Incomplete Lineage Sorting and Hybridization Statistics for Large-Scale Retroposon Insertion Data
[['In their pioneering work , Ryan and Dugaiczyk [1] first proposed using Short INterspersed Element ( SINE ) insertions as phylogenetic markers with the suggestion: “we submit that the chronology of divergence of primate lines of evolution can be correlated with the timing of insertion of new DNA repeats into the genomes of those primates” .', 'Although their originally detected insertions were of no direct phylogenetic relevance , subsequent studies fostered this innovative idea , and systematically searched for retroposon insertions as genomic landmarks of phylogeny ( e . g . [2] , [3] ) .', 'While the current most popular use of DNA sequence comparisons to deduce phylogenetic relationships must make do with only four possible character states ( ACGT ) , retroposon insertions can theoretically produce millions of different character states corresponding to the large number of random genomic insertion sites , and thereby requires special statistics to deal with such large numbers of character states .', 'Important is , that the inserted element itself does not encode the character state , but rather the character state derives from the exact genomic position of the inserted element .', 'The probabilities of two independent random insertions of the same element at the same genomic location in two unrelated lineages or the exact deletion of an orthologous element are negligible but not excludable ( see also Discussion ) .', 'For example , the probability of parallel SINE insertion in primates is calculated to be about 0 . 05% [4] and precise SINE excision to be less than 0 . 5% [5] .', 'More importantly , inexact parallel insertions or deletions are easy recognizable by careful analysis of the complex structure of each individual diagnostic element insertion , enabling these loci to be excluded from further analysis .', 'The character polarity of these markers is , in contrast to sequence data , unambiguous: presence indicates the derived state and absence the plesiomorphic condition ( for additional information on the marker system see [6] ) .', 'But it should also be mentioned , that presence/absence markers are , in contrast to sequence data , not universally available .', 'Their accumulation is not clocklike , and therefore they are not suitable for calculating exact branch-length or population size .', 'A synergistic application of both marker systems is the most efficient way to extract historical information from species .', 'An ideal phylogenetic marker evolves neutrally [7] .', 'Unfortunately , such neutral or nearly neutral markers then tend to diverge beyond recognition in relatively short times and are therefore not suitable for deep phylogenetic comparisons .', 'At the sequence analysis level , a compromise is to consider more conserved nucleotide positions ( e . g . , the second position of codons ) taking into account that such positions are less neutral and therefore may lead to only a limited phylogenetic statement .', 'On the other hand , slight natural selection rarely complicates phylogenetic analysis , as it usually involves only rate shifts , while “balancing selection” is a real challenge [8] .', 'Retroposon insertions , by contrast , are unrestricted , random , almost exclusively neutral events , and therefore virtually free of any converging effects , fulfilling essentially the strict precondition of neutral evolution [9] .', 'Due to the complex structure of inserted elements , retroposon insertions are recognizable for tens or hundreds of millions of years and are highly resistant to insertion saturation , hence resistant to post-insertional state changes .', 'The degree of natural selection on retroposon insertions correlates with the region of insertion .', 'Apart from the very rare cases of insertions into functionally significant structures ( regulatory areas , intron boundaries , or coding sequences ) , the overwhelming majority of random integrations have no functional or selective importance .', 'Any insertion , independent of where it takes place , is a unique event and post-insertional removal in a descendent lineage is easily recognizable by the highly complex traces that the insertion process leaves behind , enabling such markers to be omitted from further analysis .', 'As explained before , mutations within an element do not compromise its phylogenetic value as a unique presence/absence marker .', 'Diagnostic elements are extracted following strong criteria of orthology and only when they are clearly recognizable in all investigated lineages or when they can be irrefutably defined as absent are they used for phylogenetic analysis .', 'Another big advantage of this attractive marker system is its relative lack of conflicting data [6] .', 'When such conflicts do arise , their origins are more easily recognized than those of simple sequence changes .', 'One of the avoidable but still most common sources of apparently conflicting presence/absence patterns of retroposed elements is the violation of a strict definition of orthology .', 'In most instances of mammalian retrotranspositions , the process of insertion generates specific target site duplications ( TSD ) of 8–30 nts flanking all inserted elements [10] .', 'It is important to carefully compare the identity of such TSDs to the unoccupied site of distantly related reference species to clearly confirm the orthology of these loci .', 'The consistent orientation of inserted elements and congruent element types in all analyzed species is another essential criterion for orthology .', 'Furthermore , shared truncations of , or random indels in , elements can help to verify orthology after carefully considering potential hotspots of indels and breakpoints .', 'In the most current investigations only loci with a clear signature of presence/absence in all investigated species ( with sequence similarity >70% ) are considered [3 , 11] .', 'A second source of apparently conflicting presence/absence patterns in retrophylogenomics is incomplete lineage sorting during evolution , whereby polymorphic conditions of presence/absence states at the time of the formation of new species might lead to a random distribution of presence or absence states .', 'Such character state polymorphism can similarly influence all types of polymorphic molecular or anatomical characters .', 'Fixation starts with the appearance of an individual change in a population and continues until all individuals of the subsequent populations inherit the change , which can take several million years depending on effective population size [12] and is easily determined by t = 4Ne ( where t = generations , multiplied by 25 years for humans will lead to the estimated real time and Ne is the expected ancestral effective population size , e . g , 20 , 000 for humans ) .', 'Accordingly , for humans a fixation time of about 2 million years can be estimated .', 'Corresponding to the neutral theory of molecular evolution , the fixation of a previously polymorphic marker depends on the size of the founder population ( the smaller a population the sooner a neutral marker is fixed ) and generation time ( the shorter the generation time the sooner a marker is fixed ) .', 'For primate populations 1–3 million years are usually sufficient to fix most markers [12 , 13] .', 'Therefore , especially in rapid successive radiations and in young terminal branches , retroposed elements that entered part of a population may not yet have been uniformly fixed before the next step in speciation occurred .', 'In most such cases , this incomplete lineage sorting leads to a random presence or absence state of markers in lineages and , due to the relative unambiguity of retroposon insertions ( presence or absence ) and their insertion complexity , is more easily recognized as an equal or symmetric polytomy ( all three possible topologies of three related species are more or less equally supported ) [14] than a simple sequence change .', 'For example , the highly debated phylogenetic relationships among the three major placental branches Xenarthra , Afrotheria , and Boreotheria were intensively examined by two independent groups [15 , 16] that revealed markers for all possible variants of relationships , positive evidence supporting ancestral incomplete lineage sorting .', 'A third potential source of apparent conflicts in the presence/absence patterns of retroposed element insertions is ancestral hybridization , expressed by the exchange of genetic material between separated populations that are still able to reproduce with one another .', 'After hybridization , a new lineage or mixed old lineages can evolve that carry different amounts of genetic material from both lineages .', 'This might lead to asymmetric polytomy , as proposed for an overlapping retroposon distribution ( e . g . , two elements shared by guinea pig and squirrel vs . eight elements shared by mouse and guinea pig , but no elements shared between mouse and squirrel [17] ) .', 'Two other potential sources of conflicts , the exact deletion or parallel insertion of retroelements in related species , are both very rare ( see also above ) .', 'Lagemaat et al . [5] claimed to have found rare cases of exact deletions in young insertions with perfect recombining TSDs; however , the data are not distinguishable from those that might result from incomplete lineage sorting .', 'Notable is , that any exact deletion or exact parallel insertion ( producing the same TSDs ) in individual genomes must spread over the population to finally be fixed in a lineage .', 'So , random exact deletions or parallel insertions are very rare .', 'For LINE1-mobilized retropositions , one can recognize a slight preference for a TT/AAAA target site motif [18] ( the slash represents the cutting/insertion site ) perhaps generating some slight hotspots for insertions .', 'The distribution of such rare conflicting cases is only detectable in high-throughput computational or experimental screening for phylogenetic markers [19] .', 'At nearly the same time that insertions of SINEs were proposed as phylogenetic markers [1] , the probability of obtaining incorrect phylogenetic information due to segregation of ancestral polymorphism was intensively debated in the phylogenetic community [20] and ancestral polymorphism is now known to be common in lineage diversification [8] .', 'The first consideration of polymorphic markers was based on the principle of Kimura’s neutral theory of molecular evolution [21] .', 'However , in some of these early publications , the only source of phylogenetic conflicts considered was ancestral polymorphism due to incomplete lineage sorting [14 , 20 , 22] .', 'Recently , polymorphism due to ancestral hybridization as source for conflicting phylogenetic resolutions was discussed [23 , 24] and illustrated at the sequence analysis level [25] .', 'Notably , the probability of deriving incorrect phylogenetic signals from ancestral polymorphisms was first shown for rare and irreversible mutations [20] , which can be adapted to the analysis of presence/absence of retroelements .', 'Waddell et al . [26] created a criterion for supportive and/or conflicting SINE insertions to support or reject predefined phylogenetic topologies depending on a predefined prior hypothesis against polytomy due to incomplete lineage sorting .', 'The use of this criterion became more popular with the rising popularity of the nearly conflict-free nature of presence/absence data and the increasing availability of genomic data .', 'Nevertheless , from time to time apparently conflicting patterns were recovered and described ( e . g . , [27] ) .', 'Unfortunately , the Waddell criterion [26] has many shortcomings that are not compatible with current requirements .', 'For example , the restriction to only test trees limited to the support of five potential phylogenetically informative markers versus symmetric polytomies , or the requirement when testing experimental data that an equal amount of data must be testable for all three possible tree configurations of three species ( e . g . , for gorilla , chimpanzee , and human ideally an equal number of markers derived from all individual genomes should be screened ) is often not available from in silico data .', 'The current immense accumulation of genomic data facilitates novel multi-lineage perspectives to search for phylogenetically informative markers but also requires novel statistical models .', 'We should also note that not every phylogenetic reconstruction based on retroposon insertion presence/absence patterns is derived in an unbiased way ( e . g . , those derived from one-directional searches when just one of three genomes is available for screening; see supportable branches in red for a species A restricted search in Fig 1A–1C ) .', 'Previously , we were not able to test all possible tree topologies for those derived from one-directional searches .', 'As an example , the first systematic screenings for phylogenetically informative retroposon markers in primates [28] used the only available genome information available at the time , human .', 'Therefore , only branches leading to human could be tested and supported ( similar to the lineage leading to A in Fig 1A–1C ) .', 'Other relationships apart from the human lineage could only be examined by inspecting the few additional random insertions also present by chance in the sequenced loci .', 'The ideal situation is to independently screen for markers from two leading lineages ( see Fig 1; screening from species A and B ) to find all diagnostic insertions and potential conflicting markers .', 'To overcome the various shortcomings of previous statistical applications and to successfully analyze data that is somewhat less than ideal , we present a new statistical approach that provides a clear test system to evaluate the significance of retroposon presence/absence data and to differentiate between clear bifurcations , incomplete lineage sorting ( polytomy ) , and ancestral hybridization scenarios .', 'This tool is especially important for the high-throughput applications of current and upcoming genome projects due to the unlimited number of testable markers obtained .', 'The new differentiation for one- and multi-directional searches ( data from 1 or 2 and more leading species ) embedded in a user-friendly R-application enables us to apply the significance test to different screening strategies , and is also suitable for those cases when genomic species representation is not optimal .', 'This approach dissects phylogenetic trees into series of 3 lineages and evaluates their relationships individually with the KKSC statistics .', 'A statistical evaluation of branch support can be obtained for most such phylogenetic questions , but in the case of ancient rapid radiations leading to so-called anomaly zones with random distributions of polymorphic markers often spread over many speciation events , such a simplification will not solve conflicts between multiple groups .', 'To find phylogenetically diagnostic presence/absence insertion signals in such zones is currently impossible ( see [29] , [30] ) , because the noise ( random signals ) overlays any potential useful signal .', 'The proposed three-lineage subdivision is not adequate for such complexities , but the underlying mathematical model is being used to derive a multi-lineage application to extract hidden phylogenetic signals from a mosaic of marker information .', 'Luckily , although such anomaly zones do exist , most phylogenetic questions are simple and easy to solve with the current strategy .'], ['We consider three currently existing lineages A , B , C with a common ancestry , and inspect the presence/absence patterns for retroelements inserted at orthologous genomic loci in these lineages .', 'The following events were selected to define phylogenetically informative markers:', 'ω1—an orthologous retroelement is present in a genomic locus of A and B but absent in C;', 'ω2—an orthologous retroelement is present in a genomic locus of A and C but absent in B;', 'ω3—an orthologous retroelement is present in a genomic locus of B and C but absent in A .', 'We consider the random variable ηj as the number of events ωj ( i . e . , this variable reflects the number of presence/absence markers supporting the relatedness of two appointed lineages ) .', 'If the total number of all markers consolidating any two lineages ( n ) :', 'n=η1+η2+η3', '( 1 )', 'is fixed , then , in compliance with the proposed model ( see S1 Appendix , S1 . 8 ) , the random variables η1 , η2 , η3 are distributed according a polynomial distribution:', 'P ( η1=y1 , η2=y2 , η3=y3 ) =n !', 'y1 !', 'y2 !', 'y3 !', 'p1y1p2y2p3y3 , ( y1+y2+y3=n ) ,', '( 2 )', 'where the parameters of polynomial distribution p1 , p2 , and p3 are determined depending on which of the three models are applied , for binary branching , polytomy , and ancestral hybridization , respectively .', 'Under the term C-tree we consider a scenario where at time t0 a common ancestral population separated into two isolated branches ( that no longer interbreed ) .', 'The first branch at time T1 ( t1 = t0 + T1 ) subsequently separated into two lineages A and B . The second branch formed lineage C ( Fig 2A ) .', 'In compliance with the proposed model ( see also equations S1 . 38—S1 . 39 in S1 Appendix ) we derive:', '{p1=1−23Ψ ( τ1 ) p2=p3=13Ψ ( τ1 ) ,', '( 3 )', 'where:', 'τ1=T12N1', '( 4 )', 'is the drift time according to Waxman [41] ( see equation S1 . 14 in S1 Appendix ) , and', 'Ψ ( τ ) =e−τ1+n1n0 ( τ+e−τ−1 ) ,', '( 5 )', 'N1 is the average effective population size of the first branch before the split ( at the period [t0 , t1] ) , n1 is the average number of new insertions of retroelements per generation on this branch , and n0 is the average number of new insertions of retroelements per generation in an ancestral population .', 'It should be noted that formula ( Eq 3 ) under condition n1 = n0 coincides with the formulations obtained by Wu [20] and corrected by Hudson [22] for a phylogenetic marker system , see also Liu [14] .', 'Hence , the mathematical model for the C-tree corresponds to ( Eq 2 ) under the assumption:', 'H1={p2=p3=1−p12 , p1>13}', '( 6 )', 'Accordingly we can define the assumptions for the B-tree ( Fig 2C ) :', 'H2={p1=p3=1−p22 , p2>13}', '( 7 )', 'and the A-tree ( Fig 2B ) :', 'H3={p1=p2=1−p32 , p3>13} .', '( 8 )', 'Thus:', 'P ( η1=y1 , η2=y2 , η3=y3|Hj ) =n !', 'y1 !', 'y2 !', 'y3 !', 'pjyj ( 1−pj2 ) n−yj , pj>13 , ( y1+y2+y3=n ) .', '( 9 )', 'An ABC-tree ( polytomy ) is the extreme form of an unresolved tree topology ( Fig 2D ) :', 'H0={p1=p2=p3=13} ,', '( 10 )', 'that is:', 'P ( η1=y1 , η2=y2 , η3=y3|H0 ) =n !', 'y1 !', 'y2 !', 'y3 !', '13n , ( y1+y2+y3=n ) .', '( 11 )', 'If we assume that no other speciation scenario for A , B , and C is relevant , the parametric space for the model ( Eq 2 ) reduces to:', 'Ω=H0∪H1∪H2∪H3 .', '( 12 )', 'Thus , to accept for example hypothesis H1 , we must reject the opposite hypothesis:', 'H023=H0∪H2∪H3 .', '( 13 )', 'This leads to the fact that the data relevant for rejecting hypothesis H023 are at the same time sufficient for automatically accepting H1 .', 'An example result [27:13:0] representing relevant markers for the A , B , and C trees accordingly , will contradict the assumptions ( Eq 7 ) , ( Eq 8 ) , and ( Eq 10 ) with a clear significance at the 5% level ( in fact , even higher ) .', 'This corresponds to Wu [20] .', 'However , this result will also be inconsistent with ( Eq 6 ) for the last two numbers [13:0] ( for B and C trees ) .', 'This indicates significant differences between p2 and p3 ( that should be equal ) that cannot be explained in either the present or previous models [20 , 22 , 26] or for coalescence models [14] .', 'However , the skewed distribution of markers ( e . g . , 0 vs . 13 ) can be explained by ancestral hybridization [23 , 24] .', 'To accommodate this , we added a simple model of hybridization that allows any combination of values of p1 , p2 , and p3 , including the binary trees as a special case ( see equations S1 . 40-S1 . 60 in S1 Appendix ) .', 'For ancestral hybridization ( Fig 2E ) we assume that at time t = t0 the common ancestral population separated into two isolated branches .', 'Later , after T1 and T2 generations , subpopulations of each of the two branches separated from their parent branches ( indicated by vertical lines on Fig 2E ) and reproduce with one another , forming a new branch B ( horizontal line , respectively; Fig 2E ) .', 'The remaining two branches represent lineages A and C ( Fig 2E ) .', 'We will call this scenario B-fusion .', 'In this simple scenario we ignore all events in the subpopulations before fusion , because elements inserted in genomes on these branches do not generate informative data .', 'The proportions of the two subpopulations in the newly joined population are denoted by γ1 and γ2 ( γ1 + γ2 = 1 ) .', 'Then , according to the proposed mathematical model ( equation S1 . 57 in S1 Appendix ) , if γ1 , 2 is not equal to 0 or 1 we have:', 'p1>p2andp3>p2 .', '( 14 )', 'When either γ1 or γ2 is equal to 0 , we obtain an A-tree or C-tree , respectively .', 'In the case of C-fusion ( splits from A and B fuse ) , p1 exchanges places with p2 , and in the case of A-fusion ( splits from B and C fuse ) , p3 exchanges places with p2 .', 'Consider the C-tree hypothesis:', 'H1={p2=p3=1−p12 , p1>13} .', '( 15 )', 'In fact , this is equivalent to the two statements:', 'H1+={p1>13}andH23={p2=p3} .', '( 16 )', 'Therefore , H1 is accepted when both hypotheses ( H1+ and H23 ) are supported and rejected when at least one of them is not accepted .', 'In turn , the hypothesis H1+ is accepted when its opposite hypothesis', 'H¯1+={p1≤13}', 'is rejected .', 'η1 is a sufficient statistic for the parameter p1 , and distributes according to the binomial distribution:', 'P ( η1=k ) = ( nk ) p1k⋅ ( 1−p ) n−k ,', '( 17 )', 'where', '( nk ) =n !', 'k !', '( n−k ) !', '.', 'Thus , if we obtain η1 = Y1 , the critical region for the hypothesis', 'H¯1+', 'is the set of values greater or equal to Y1 .', 'Then:', 'P ( η1≥Y1 ) =∑k=Y1n ( nk ) p1k⋅ ( 1−p ) n−k=Ip1 ( Y1 , n−Y1+1 ) ,', '( 18 )', 'where Ip ( x , y ) is an incomplete beta function , which can also be expressed by the cumulative binomial distribution function:', 'Pbinom ( m , n , p ) =∑k=0m ( nk ) pk⋅ ( 1−p ) n−k=1−Ip ( m+1 , n−m ) .', '( 19 )', 'Thus , the significance level is defined by the formula:', 'SL1 ( Y ) =max︸p≤13P ( η1≥Y1 ) =I13 ( Y1 , n−Y1+1 ) .', '( 20 )', 'We define the maximum probability of a Type I Error α as the probability to reject', 'H¯1+', 'in favor of H1+ when', 'H¯1+', 'is true .', 'Thus , if SL1 ( Y ) ≤ α , then hypothesis', 'H¯1+', 'is rejected , and hypothesis H1+ is accepted .', 'Note , that when testing the hypothesis H23 , the conditional distribution of the random variable η2 is binomially distributed with the parameter', 'p=p2p2+p3', ':', 'P ( η1=k|η2+η3=m ) = ( mk ) pk⋅ ( 1−p ) m−k ,', '( 21 )', 'and hypothesis H23 is equivalent to the statement:', 'p=12', '.', 'When using a two-sided test , the test statistics will be max{η2 , η3} .', 'In the case that the experimental data is validated ( η2 = Y2 , η3 = Y3 ) , the critical region for the hypothesis H23 is the set of values {y2 + y3 = Y2 + Y3 , max{y2 , y3} ≥ max{Y2 , Y3}} .', 'Accordingly , the level of significance is:', 'SL23 ( Y ) =|2I12 ( max{Y2 , Y3}1 , min{Y2 , Y3}+1 ) , ifY2≠Y3 , 1 , ifY2=Y3 ,', '( 22 )', 'An illustration of all outcomes for the random distribution of markers and significance areas is presented in Fig 3 .', 'In the case of a one-directional search for markers , data are only available to support two configurations of trees , while four configurations of trees and three hybridization scenarios are possible .', 'This information is insufficient for our full model , but for the condition in which hybridization has already been ruled out , and we assume that only bifurcating trees or polytomy are possible , we derived a simple model for comparing two random binomial-distributed variables ( see equations S1 . 61—S1 . 63 in S1 Appendix ) .', 'We consider the random binomial-distributed variables η1 and η2 and testing of hypothesis', 'H0:p1≤12', '( B-tree , A-tree , ABC-tree ) against an alternative hypothesis', 'H1:p1>12', '( C-tree ) .', 'Then , when H0 is rejected because Y1 is significantly bigger than Y2 , the C-tree can be accepted .', 'In the opposite case , when Y2 > Y1 , we can reject', 'H0:p2≤12', '( C-tree , A-tree , ABC-tree ) and accept the B-tree .', 'If Y1 and Y2 are empirical values of η1 and η2 , then , calculating similarly to ( Eq 22 ) , the level of significance will be:', 'SL ( Y ) =I12 ( Y1 , Y2+1 )', '( 23 )', '( Except in the situation where Y1 = Y2 and H0 is certainly accepted ) .', 'The direct calculation of probabilities for large sets of phylogenetic markers requires some extensive calculations and extended knowledge of mathematical functions .', 'Approximations can help to derive computational scripts including the statistical test .', 'To find the boundaries of critical areas , we can use the normal approximation:', 'P ( η1≥Y1 ) ≈1−F0 ( Y1−12−np1np1 ( 1−p1 ) ) ,', '( 24 )', 'where F0 ( x ) is the standard normal distribution function .', 'Denoting zα as the root of the equation F0 ( z ) = 1−α , from the condition P ( η1 ≥ Y1 ) ≤ α and assuming', 'p1=13', 'we obtain:', 'Y1≥n3+12+zα2n3 .', '( 25 )', 'Proceeding similarly , we define the second critical area for a given level of significance α as:', '|Y2−Y3|≥1+zα2Y2+Y3 .', '( 26 )', 'In the case of a one-sided comparison ( 23 ) , the critical area is defined by the formula:', 'Y1≥Y1+Y2+1+zαY1+Y22 .', '( 27 )', 'Values for zα used in the approximated formulas ( 25 ) , ( 26 ) and ( 27 ) are given in Table S1 in S2 Appendix for significance levels α<0 . 05 , α<0 . 025 , α<0 . 01 , and α<0 . 005 .', 'The statistical model described here was implemented in a graphical web-application available at http://retrogenomics . uni-muenster . de:3838/KKSC_significance_test/ .', 'The application is generated with the Shiny package [32] in the R language [33] .', 'No additional software needs to be installed to use it .'], ['Based on our proposed mathematical model presented in the Methods ( see also S1 Appendix ) , we can calculate the critical values for the numbers of markers shared by two lineages for various schemes of phylogenetic studies .', 'A one-directional search ( genome information of only one of three species is available; e . g . , Fig 1A–1C for species A ) provides a very limited amount of interpretable information .', 'The calculation is based on formulas ( Eqs 23 and 27 ) ( see Table S2 in S3 Appendix ) .', 'However , interpretations of presence/absence patterns derived from one-directional searches should be made with care .', 'The lack of a difference between two values ( numbers of markers ) does not necessarily reject the third possible tree configuration , which cannot be tested from this one direction , and cannot exclude a polytomy between all three possible configurations or a significantly resolved third tree hypothesis ( e . g . , Fig 1F; the genome of species B is necessary ) .', 'On the other hand , based on our model , differences between the two smallest values indicate ancestral hybridization events .', 'Then significant statistical differences between two values obtained in the one-directional search do not distinguish between the possible bifurcated tree and hybridization ( see equations S1 . 61—S1 . 63 in S1 Appendix ) .', 'In contrast to one-directional searches , unbiased screenings ( multi-directional search ) from two directions ( e . g . , Fig 1 using genomes of species A and B ) , returning three values for the numbers of shared markers , provide more information for interpretation ( Tables S3-S4 in S3 Appendix ) , based on our statistical two-step criterion ( Eqs 22–23 ) .', 'Using our web-interface and the implemented model ( Eqs 20 and 22 and 23 ) , we can easily derive P-values for the different phylogenetic scenarios ( http://retrogenomics . uni-muenster . de:3838/KKSC_significance_test/ ) .', 'An example of a conflicting distribution of markers was detected when we inspected the root of placental mammals [15] .', 'We identified a presence/absence pattern of ( 9:8:5 ) similarly supporting all three possible tree hypotheses ( Epitheria , Atlantogenata , and Exafroplacentalia ) .', 'Using the web application to resolve this contradiction , the user first selects the “Analysis type” , either a “multi-directional” search ( for cases in which more than one reference genome were screened , as in this example ) , or a “one-directional” search ( for cases in which a screening was performed from only one reference species ) .', 'It is also possible to specify the names of the species ( e . g . , A: Afrotheria; B: Xenarthra; C; Boreotheria ) , which are used for the results table .', 'The user then provides the numbers of markers shared by the lineages that were analyzed .', 'For the current example of a multi-directional analysis , the Afrotheria and Xenarthra shared 8 markers , Xenarthra and Boreotheria shared 5 , and Afrotheria and Boreotheria 9 markers .', 'The table at http://retrogenomics . uni-muenster . de:3838/KKSC_significance_test/ displays statistical information about the tests .', 'The column “test type” displays the type of the test , and P-values are calculated for the different tests based on the values presented in the third column ( e . g . , p = 0 . 5811 for the hybridization test and p = 0 . 293 for bifurcation test ) .', 'The fourth and fifth columns display the boundaries of critical areas for p<0 . 05 and p<0 . 01 , respectively .', 'The resulting figure of the KKSC significance test highlights the most probable evolutionary scenario .', 'Significantly supported lineages are labeled by dark spheres; hybridization is indicated by divided spheres labeled with the hybridizing lineages; and the tree located in the center of the triangle indicates an unresolved tree topology .', 'We have also presented an applicable approximation for an unlimited number of markers ( Eqs 25–27 ) .', 'As can be seen in Tables S3-S4 in S3 Appendix ( columns 5% and 1% borders ) , this approximation effectively works from the minimum number of markers , and can be used as a brief estimation of significance of ongoing experimental results without using tables or the web-interface .', 'For example , Nishihara et al . [16] examined the root of the placental tree and found 25 retroposon insertions supporting the Epitheria hypothesis , 22 supporting the Exafroplacentalia hypothesis , and 21 supporting the Atlantogenata hypothesis .', 'Because the total number of markers is larger than 30 , the pattern ( 25:22:21 ) cannot be directly evaluated using Tables S3-S4 in S3 Appendix .', 'Therefore , to test the significance of the support for the various hypotheses the approximation formulas or the web-interface ( http://retrogenomics . uni-muenster . de:3838/KKSC_significance_test/ ) should be used .', 'To test the Epitheria hypothesis: calculate the sum of the relevant supporting markers ( 22+21 = 43 ) and the difference of the two smallest values ( 22–21 ) .', 'Setting the significance level at α<0 . 05 , from Table S1 in S2 Appendix , we have a value of', 'zα2=1 . 960', '.', 'Using equation ( Eq 26 ) we can calculate the critical value for the difference of the two smallest values for their sum 43 and round this value up to the closest integer value:', '1+1 . 960⋅43≈13 . 9=⊳14', '.', 'Thus , on the level of a significance of α<0 . 05 , we cannot accept the hybridization hypothesis .', 'To test the Epitheria hypothesis against polytomy we calculate the full sum ( n = 25+22+21 = 68 ) and use equation ( Eq 25 ) .', 'Setting the significance level at α<0 . 05 , from Table S1 in S2 Appendix we have a value of zα = 1 . 645 .', 'Calculating the critical value and rounding up , we have:', '0 . 5+68+1 . 645⋅2⋅683≈29 . 6=⊳30', '.', 'Then , because 30<33 , polytomy cannot be rejected and should represent the most realistic evolutionary scenario .', 'We also analyzed an interesting example of asymmetric conflicts in rodents .', 'To determine the origin of the three major rodent lineages , best represented by mouse , guinea pig , and squirrel [16] , we found 8 markers shared by mouse and guinea pig to the exclusion of squirrel , but also two markers shared by guinea pig and squirrel to the exclusion of mouse , and no insertions shared by mouse and squirrel .', 'Because the Waddell criterion i
['Introduction', 'Methods', 'Results', 'Discussion']
['Ancient retroposon insertions can be used as virtually homoplasy-free markers to reconstruct the phylogenetic history of species .', 'Inherited , orthologous insertions in related species offer reliable signals of a common origin of the given species .', 'One prerequisite for such a phylogenetically informative insertion is that the inserted element was fixed in the ancestral population before speciation; if not , polymorphically inserted elements may lead to random distributions of presence/absence states during speciation and possibly to apparently conflicting reconstructions of their ancestry .', 'Fortunately , such misleading fixed cases are relatively rare but nevertheless , need to be considered .', 'Here , we present novel , comprehensive statistical models applicable for ( 1 ) analyzing any pattern of rare genomic changes , ( 2 ) testing and differentiating conflicting phylogenetic reconstructions based on rare genomic changes caused by incomplete lineage sorting or/and ancestral hybridization , and ( 3 ) differentiating between search strategies involving genome information from one or several lineages .', 'When the new statistics are applied , in non-conflicting cases a minimum of three elements present in both of two species and absent in a third group are considered significant support ( p<0 . 05 ) for the branching of the third from the other two , if all three of the given species are screened equally for genome or experimental data .', 'Five elements are necessary for significant support ( p<0 . 05 ) if a diagnostic locus derived from only one of three species is screened , and no conflicting markers are detected .', 'Most potentially conflicting patterns can be evaluated for their significance and ancestral hybridization can be distinguished from incomplete lineage sorting by considering symmetric or asymmetric distribution of rare genomic changes among possible tree configurations .', 'Additionally , we provide an R-application to make the new KKSC insertion significance test available for the scientific community at http://retrogenomics . uni-muenster . de:3838/KKSC_significance_test/ .']
['The presence/absence patterns of transposed elements , so called jumping genes , provide invaluable information about evolution .', 'Unfortunately , there is still no clear all-encompassing analysis of the statistical significance of insertion patterns , and the single existing model of insertion data is no longer sufficient for the emerging genomic era .', 'Here , we have provided a comprehensive statistical framework for testing the significance of support for phylogenetic hypotheses derived from genome-level presence/absence data such as retroposon insertions and for evaluating such data for different evolutionary scenarios , including polytomy , incomplete lineage sorting , and ancestral hybridization .', 'This statistical framework is especially important for high-throughput applications of current and upcoming genome projects due to its treatment of unlimited numbers of testable markers , and is embedded in a user-friendly R-application available to the scientific community online .', 'Finally , a reliable , adaptable calculation for the significance of support for phylogenetic trees derived from retroposon presence/absence data is now available .']
['taxonomy', 'marker genes', 'animal phylogenetics', 'phylogenetics', 'data management', 'mathematics', 'statistics (mathematics)', 'phylogenetic analysis', 'paleontology', 'molecular biology techniques', 'mammalian genomics', 'paleogenetics', 'zoology', 'research and analysis methods', 'selection markers', 'computer and information sciences', 'hybridization', 'evolutionary systematics', 'molecular biology', 'animal genomics', 'molecular biology assays and analysis techniques', 'earth sciences', 'genetics', 'biology and life sciences', 'physical sciences', 'genomics', 'evolutionary biology', 'evolutionary processes', 'statistical data']