the Zn-Te bond in zinc blende ZnTe has greater ionic character than that in wurtzite ZnTe. Due to these differences it has been shown from ab-initio calculations that the band gaps of the structures should be different. Similarly, it is also expected that the electronic properties will be different. There are, however, very limited studies on the growth and stabilization of wurtzite ZnTe films. The present authors have grown wurtzite ZnTe films by electron beam evaporation of a wurtzite ZnTe source material. Typical x-ray diffraction patterns of the wurtzite films are shown in fig.8 for different thicknesses. The measured spectral transmittance for the zinc blende and wurtzite films are shown in fig.9. Detailed dispersion analysis of the films and comparison with zinc blende films has been carried out. ZnTe thin films exhibit either the wurtzite or zinc blende structure depending on the chemical nature of the source material. The formation of the structural phases is confirmed by a combination of x-ray diffraction and selected area electron diffraction. The films were nanocrystalline and randomly textured. The refractive index of the wurtzite phase (3.87 at 2000 nm) is higher than that for the zinc blende phase (3.03 at 2000 nm). The optical band gap, however, is lower for the wurtzite phase (1.1 eV) than the zinc blende phase (2.26 eV). The optical dispersion behavior has been investigated within the framework of the single effective oscillator model. It is shown that the oscillator strength, dispersion energy and optical charge carrier concentrations are all lower for the wurtzite phase than the zinc blende phase.

Fig. 8. XRD patterns of wurtzite ZnTe films of different thickness

Fig. 9. Comparison of optical response of zinc blende and wurtzite ZnTe thin films.

Fig. 6. X-ray diffraction patterns of the ZnTe films of different thicknesses showing the formation of zinc blende structure.

The measured spectral transmission curves for zinc blende ZnTe films of thicknesses upto 1200 nm and typical x-ray diffraction pattern of the 1200 nm film is shown in figs. 5 and 6 respectively. The transmission of the films is close to 90% in the near IR region and calculated refractive indices were between 2.0 and 2.6. The optical band gaps were between 2 and 2.2 eV. These values compared favourably with earlier reports.Significantly the 1200 nm thick film was nanocrystalline ( with a crystallite size of 24 nm) and showed a preferred (111) orientation. The thickest films achieved using this technique had a thickness of 10-12 microns

The atomic force microscope image of a 2000 nm thick zinc blende ZnTe film is shown in fig. 7. The film is clearly very smooth with roughness less than 10 nm even at this thickness. The grain sizes are of the order of 100 nm.

Fig. 7. Atomic force microscope image of a 2000 nm zinc blende ZnTe film.

ZnTe also crystallizes in the wurtzite structure wherein the stacking sequence of the cation and anion is ABABABAB. . . with each repeated period perpendicular to the basal plane, whereas in the zinc blende structure the sequence is ABCABCABC. Therefore, the distance between the third nearest-neighbor (NN) atoms is small in wurtzite ZnTe. In contrast, the distance between the first and second NN atoms is invariable in both structures. In addition,

Fig. 6. X-ray diffraction patterns of the ZnTe films of different thicknesses showing the

The thickest films achieved using this technique had a thickness of 10-12 microns

Fig. 7. Atomic force microscope image of a 2000 nm zinc blende ZnTe film.

ZnTe also crystallizes in the wurtzite structure wherein the stacking sequence of the cation and anion is ABABABAB. . . with each repeated period perpendicular to the basal plane, whereas in the zinc blende structure the sequence is ABCABCABC. Therefore, the distance between the third nearest-neighbor (NN) atoms is small in wurtzite ZnTe. In contrast, the distance between the first and second NN atoms is invariable in both structures. In addition,

The measured spectral transmission curves for zinc blende ZnTe films of thicknesses upto 1200 nm and typical x-ray diffraction pattern of the 1200 nm film is shown in figs. 5 and 6 respectively. The transmission of the films is close to 90% in the near IR region and calculated refractive indices were between 2.0 and 2.6. The optical band gaps were between 2 and 2.2 eV. These values compared favourably with earlier reports.Significantly the 1200 nm thick film was nanocrystalline ( with a crystallite size of 24 nm) and showed a preferred (111) orientation.

The atomic force microscope image of a 2000 nm thick zinc blende ZnTe film is shown in fig. 7. The film is clearly very smooth with roughness less than 10 nm even at this thickness. The

formation of zinc blende structure.

grain sizes are of the order of 100 nm.

the Zn-Te bond in zinc blende ZnTe has greater ionic character than that in wurtzite ZnTe. Due to these differences it has been shown from ab-initio calculations that the band gaps of the structures should be different. Similarly, it is also expected that the electronic properties will be different. There are, however, very limited studies on the growth and stabilization of wurtzite ZnTe films. The present authors have grown wurtzite ZnTe films by electron beam evaporation of a wurtzite ZnTe source material. Typical x-ray diffraction patterns of the wurtzite films are shown in fig.8 for different thicknesses. The measured spectral transmittance for the zinc blende and wurtzite films are shown in fig.9. Detailed dispersion analysis of the films and comparison with zinc blende films has been carried out. ZnTe thin films exhibit either the wurtzite or zinc blende structure depending on the chemical nature of the source material. The formation of the structural phases is confirmed by a combination of x-ray diffraction and selected area electron diffraction. The films were nanocrystalline and randomly textured. The refractive index of the wurtzite phase (3.87 at 2000 nm) is higher than that for the zinc blende phase (3.03 at 2000 nm). The optical band gap, however, is lower for the wurtzite phase (1.1 eV) than the zinc blende phase (2.26 eV). The optical dispersion behavior has been investigated within the framework of the single effective oscillator model. It is shown that the oscillator strength, dispersion energy and optical charge carrier concentrations are all lower for the wurtzite phase than the zinc blende phase.

Fig. 8. XRD patterns of wurtzite ZnTe films of different thickness

Fig. 9. Comparison of optical response of zinc blende and wurtzite ZnTe thin films.

Terahertz Emitters, Detectors and Sensors: Current Status and Future Prospects 137

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The charge carrier concentration in wurtzite phase (5 x1025m-3) is lower than the zinc blende structure (4.17 x 1026 m-3) This may make the wurtzite phase suitable for the development of *n*-type ZnTe, which has been a long standing challenge. Evidently the wurtzite phase has the potential to be employed as an n-type material leading to the formation of ZnTe p-n junctions for THz applications. Further developments in this direction are ongoing

Our work thus demonstrates the growth of thick ZnTe coatings in both the zinc blende and wurtzite polymorphs. Their optical properties are substantially different, signifying the possibility of applications in different frequencies regimes of the THz region [Kiran et al, 2010; Kshirsagar et al, 2011].

### **6. Conclusions and future prospects**

In summary, a number of technologies for THz emitters, detectors and sensors have been reviewed. It is obvious that, while much progress has been made in all three areas, there is scope for new science and technology to emerge in all these areas. Significantly, the progress in the area of THz electronics has been much more rapid than that in the area of THz photonics. This is mainly due to the lack of cost effective so-called "table top" femto second laser sources. THz electronics, on the other hand, relies solely on existing mature technologies for materials and device fabrication. Much of the materials development has focused on single crystals. However, for miniaturization of THz technologies (Photonics and electronics) to succeed, the area of coatings, films and micro-nano devices has to be focused upon, in addition to development of compact THz sources. A report on the development of a high-resolution terahertz (THz) imaging beyond the diffraction limit by using a twodimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure, points to effort in this direction[.Kawano & Ishibashi, 2010]. In the area of materials, strongly correlated electron systems have shown great potential[Ge et al, 2010]. In addition, novel device structures to exploit the plasma instability for THz emission and detection is another area that is receiving greater attention. Interestingly, access to fs laser sources has led to better understanding of physical and chemical phenomena in the THz region, that will eventually lead to new materials and device structures. Two examples exemplify this theme. It has recently been shown [Chan et al., 2011] that a nano mechanical oscillator coupled to single mode electromagnetic oscillator in a nano optical cavity can be designed to have oscillations in THz frequency domain. The physics of this domain of energy storage as envisaged by the Debye model has to be examined and the modes of vibrations at phase boundaries, and domain boundaries studied through THz experiments. These will reveal new materials and methods of generation and detection and sensing in the THz range. In another report [Chen et al, 2011], impulsive stimulated Raman scattering and Time-domain THz spectroscopy of coherent optical phonons in bismuth germinate (Bi4Ge3O12) revealed more than 12 unique vibrational states ranging in frequency from 2 to 11 THz, each with coherent lifetimes ranging from 1 to 20 ps. Such studies on other systems can be expected have a profound impact on the developments in THz sources, emitters, detectors and sensors.

#### **7. Acknowledgements**

The authors acknowledge the facilities provided by the School of Physics under the UGC-UPE and CAS programs. A Dr D S Kothari Fellowship for SK is also acknowledged.

### **8. References**

136 Photodetectors

The charge carrier concentration in wurtzite phase (5 x1025m-3) is lower than the zinc blende structure (4.17 x 1026 m-3) This may make the wurtzite phase suitable for the development of *n*-type ZnTe, which has been a long standing challenge. Evidently the wurtzite phase has the potential to be employed as an n-type material leading to the formation of ZnTe p-n

Our work thus demonstrates the growth of thick ZnTe coatings in both the zinc blende and wurtzite polymorphs. Their optical properties are substantially different, signifying the possibility of applications in different frequencies regimes of the THz region [Kiran et al,

In summary, a number of technologies for THz emitters, detectors and sensors have been reviewed. It is obvious that, while much progress has been made in all three areas, there is scope for new science and technology to emerge in all these areas. Significantly, the progress in the area of THz electronics has been much more rapid than that in the area of THz photonics. This is mainly due to the lack of cost effective so-called "table top" femto second laser sources. THz electronics, on the other hand, relies solely on existing mature technologies for materials and device fabrication. Much of the materials development has focused on single crystals. However, for miniaturization of THz technologies (Photonics and electronics) to succeed, the area of coatings, films and micro-nano devices has to be focused upon, in addition to development of compact THz sources. A report on the development of a high-resolution terahertz (THz) imaging beyond the diffraction limit by using a twodimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure, points to effort in this direction[.Kawano & Ishibashi, 2010]. In the area of materials, strongly correlated electron systems have shown great potential[Ge et al, 2010]. In addition, novel device structures to exploit the plasma instability for THz emission and detection is another area that is receiving greater attention. Interestingly, access to fs laser sources has led to better understanding of physical and chemical phenomena in the THz region, that will eventually lead to new materials and device structures. Two examples exemplify this theme. It has recently been shown [Chan et al., 2011] that a nano mechanical oscillator coupled to single mode electromagnetic oscillator in a nano optical cavity can be designed to have oscillations in THz frequency domain. The physics of this domain of energy storage as envisaged by the Debye model has to be examined and the modes of vibrations at phase boundaries, and domain boundaries studied through THz experiments. These will reveal new materials and methods of generation and detection and sensing in the THz range. In another report [Chen et al, 2011], impulsive stimulated Raman scattering and Time-domain THz spectroscopy of coherent optical phonons in bismuth germinate (Bi4Ge3O12) revealed more than 12 unique vibrational states ranging in frequency from 2 to 11 THz, each with coherent lifetimes ranging from 1 to 20 ps. Such studies on other systems can be expected have a profound

junctions for THz applications. Further developments in this direction are ongoing

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The authors acknowledge the facilities provided by the School of Physics under the UGC-

UPE and CAS programs. A Dr D S Kothari Fellowship for SK is also acknowledged.

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**7**

*Japan*

**Far-Infrared Single-Photon Detectors Fabricated**

Sensitive infrared (IR) detectors are key elements for both fundamental research and applications, and development of sensitive IR detectors is one of intensive research subjects in resent years (Rogalski, 2002; Tidrow, 2000). The activities are motivated by both basic and applied research fields (Prochazka, 2005; Rogalski, 2002). For certain applications, an ultimately high sensitivity reaching a photon-counting level is indispensable; for instance, passive high-resolution microscopy, catching IR photons spontaneously emitted by a living cell or a small number of biomolecules, may be accessible only with such ultimately

Photon-counters are routinely used in visible and near infrared regions (Prochazka, 2005). In the long-wavelength range (*λ >* 10 *μ*m, frequencies lower than 30 THz), called terahertz (THz) region, however, photon energies are far smaller (*hν <* 124 meV for *λ >* 10 *μ*m) and the single-photon detection is no longer trivial. Nevertheless, the THz region is one of the richest areas of spectroscopy of matters, encompassing the rotational spectra of molecules, vibrational spectra of molecules, liquids and solids, and the electron energy spectra in semiconductor nanostructures and superconducting energy gap in metals. Hence, sensitive microscopy of matters with high spatial resolution in this spectral region would provide a unique powerful tool for investigation of matters. Sensitive observation is also strongly

In the last decade, a variety of novel detection schemes have been proposed (Astaviev et al., 2002; Day et al., 2003; Hashiba et al., 2006; Komiyama et al., 2000; Schoelkopf et al., 1999; Wei et al., 2008). Among them, only semiconductor quantum devices have demonstrated single-photon detection (Astaviev et al., 2002; Hashiba et al., 2006; Komiyama et al., 2000): The experimentally achieved noise equivalent power (NEP), less than 1 *×* 10−19 W/Hz1*/*2, is

These Semiconductor photon-counters exploit a novel scheme, in which the photon absorption event leads to generation of a long-lived unit charge that is probed by a singleelectron transistor (SET). The use of SETs, however, might restrict their application because the operation is limited to ultra-low temperatures (<1K). From the viewpoint of broader application, therefore, photon-counters usable at elevated temperatures are highly desirable. From the viewpoint of importance of the spectra of matters, it is also extremely important to

by several orders of magnitude lower than those of any other detectors.

**1. Introduction**

sensitive detectors.

demanded in astrophysics (Nakagawa et al, 2007).

**in Double-Quantum-Well Structures**

Takeji Ueda and Susumu Komiyama

*The University of Tokyo*

induced optical rectification, *Physica B: Condensed Matter,* Vol.398, No. 98 (May 2007), pp. 98-101, ISSN 0921-4526


**4. General anesthesia and ANS: control and management in neurosurgery**

### **4.1 Anesthesia depth control (BIS monitoring)**

The key anatomic structures of the central nervous system (CNS) that contribute to the state of consciousness are—the brain stem, the pons, the thalamus (thalamic nuclei), and the brain cortex with their connecting neural pathways [56].

General anesthetics (propofol, sevoflurane, desflurane) inhibit the excitatory arousal pathways originating in the brain stem and pons or potentiate the sleep pathways that control them [57]. The brain stem and pontine nuclei have been known to be essential in maintaining cortical arousal and forming the so-called ascending reticular formation [57].

The most important task of an anesthesiologist during neurosurgical operation is to keep anesthesia depth sufficient for security vegetative stability of the patient

*General Anesthesia and Autonomic Nervous System: Control and Management in Neurosurgery DOI: http://dx.doi.org/10.5772/intechopen.101829*

### **Figure 5.**

*BIS-monitoring (rose line, BIS = 36) during general anesthesia (TIVA) in patient with brain tumor removal (the image is taken from the private archive of I. A. Savvina).*

under the condition of general anesthesia during neurosurgical manipulations especially on the brain stem anatomic structures, cerebral arteries (clipping of cerebral aneurysm), etc. For this purpose, it is necessary to use BIS monitoring. The main component of the BIS monitor is the bispectral analysis, which evaluates the phase relations from a single-channel EEG signal measured from the patient's forehead. The BIS index is a dimensionless number from 0 to 100. For neurosurgery, the optimal means of BIS index is around 40 so during total intravenous anesthesia (TIVA with propofol, opioid μ-agonist fentanyl) as inhalational general anesthesia (sevoflurane, desflurane). Control of the depth of TIVA during brain tumor removal (giant trigeminal schwannoma) is reflected in **Figure 5**.

### **4.2 Intraoperative antinociceptive defense**

Nociceptive stimulation during neurosurgical interventions on the brain, spine and spinal cord, peripheral nerves triggers activation of the sympathetic link of the autonomic nervous system, aseptic systemic inflammatory response [29, 34, 58]. As you know, the actual "pain" receptors are located in the skin, periosteum, and dura mater. Anticipating the development of such a scenario is possible when drugs from the NSAID group are included in the premedication scheme, in particular, ketoprofen, ketonal, lornoxicam [59]; the use of the technique of locoregional anesthesia in neurosurgery [60, 61]. There are both proponents of this approach and its opponents who associate the administration of NSAIDs in the perioperative period in neurosurgical patients with their negative effect on platelet aggregation and additional risks of postoperative hemorrhagic complications [59].

The concept of multimodal multicomponent analgesia in neurosurgical practice finds successful implementation in the form of proactive administration of NSAIDs at the stage of premedication (before the skin incision), locoregional anesthesia using naropin, at the stage of induction and maintenance of anesthesia—opioid analgesic (fentanyl) and α2-adrenergic agonist (clonidine or dexmedetomidine), when suturing a skin wound—paracetamol [53, 62–64].

On the main stage of the neurosurgical operation (brain tumor removing, cerebral aneurysm clipping, spinal hord tumor, AVM removing, etc.) antinociceptive defense is carried out with opioid analgesic μ-agonist fentanyl 3.5–5.0 mcg/kg/h + α2-adrenergic agonist clonidine 0.5 mcg/kg/h or dexmedetomidine 0.2–0.4 mcg/kg/h.

Neurovegetative stabilization with such a method and doses of drugs will be sufficient for cupping of central hemodynamic reactions during neurosurgery [65–69].

## **4.3 Hemodynamic monitoring**

Hemodynamic monitoring is necessary and mandatory part of intraoperative monitoring according to Helsinki Declaration on Patient Safety in Anesthesiology (2010). Noninvasive, invasive hemodynamic monitoring, on the base of technology "PiCCO,"—all options are used in neurosurgery—arterial blood pressure systolic, diastolic, mean (BP), cardiac index (CI), stroke index (SI), global end-diastolic volume index (GEDVI), stroke volume variability (SVV), left ventricular contractility index (LVCI), total peripheral vascular resistance index (TPVRI), intrathoracic blood volume index (ITBVI), extravascular lung water index (EVLWI), pulmonary vascular permeability index (PVPI) [67, 69–71]. The choice of kind and volume of hemodynamic monitoring in neurosurgery is determined by the patient's condition (ASA classification), operation position, localization of brain tumor, supposed blood loss, etc. The level of neurovegetative stabilization is controlled on the basis of the evaluation of the hemodynamic profile of the patient and depth of anesthesia (**Figure 6**) [71].


**1. Introduction**

The adaptive function of sex remains, today, one of the major unsolved problems in biology. Fundamental to achieving a resolution of this problem is gaining an understanding of the function of meiosis. The sexual cycle in eukaryotes has two key stages, meiosis and syngamy. In meiosis, typically a diploid cell gives rise to haploid cells. In syngamy (fertilization), typically two haploid gametes from different individuals fuse to generate a new diploid individual. A unique feature of meiosis, compared to mitosis, is recombination between non-sister homologous chromosomes. Usually these homologous chromosomes are derived from different individuals. In mitosis, recombination can occur, but it is ordinarily between sister homologs, the two products of a round of chromosome replication. Birdsell & Wills (2003) have reviewed the various hypotheses for the origin and maintenance of sex and meiotic recombination, including the hypothesis that sex is an adaptation for the repair of DNA damage and the masking of deleterious recessive alleles. Recently, we presented evidence that among microbial pathogens, sexual processes promote repair of DNA damage, especially when challenged by the oxidative defenses of their biologic hosts (Michod et al., 2008). Here, we present evidence that meiosis is primarily an evolutionary adaptation for DNA repair. Since our previous review of this topic (Bernstein et al., 1988), there has been a considerable increase in relevant information at the molecular level on the DNA repair functions of meiotic recombination, and this new information is emphasized in the present chapter.

#### **2. Meiosis in protists and simple multicellular eukaryotes is induced in response to stressful conditions that likely cause DNA damage**

Eukaryotes appeared in evolution more than 1.5 billion years ago (Javaux et al., 2001). Among extant eukaryotes, meiosis and sexual reproduction are ubiquitous and appear to have been present early in eukaryote evolution. Malik et al. (2008) found that 27 of 29 tested meiotic genes were present in *Trichomonas vaginalis*, and 21 of these 29 genes were also present in *Giardia intestinalis*, indicating that most meiotic genes were present in a common ancestor of these species. Since these lineages are highly divergent among eukaryotes, these authors concluded that each of these meiotic genes were likely present in the common ancestor of all eukaryotes. Dacks and Roger (1999) also proposed that sex has a single

Meiosis as an Evolutionary Adaptation for DNA Repair 359

These observations suggest that meiosis is an adaptation for dealing with stress, particularly oxidative stress. It is well established that oxidative stress induces a variety of DNA damages including DNA DSBs, single-strand breaks and modified bases (Slupphaug et al., 2003). Thus we hypothesize that, in facultative sexual protists and simple multicellular eukaryotes, sex, with the central feature of meiosis, is an adaptive response to DNA damage,

**3. DNA damages induced by exogenous agents cause increased meiotic**

If recombination during meiosis is an adaptation for repairing DNA damages, then it would be expected that exposure to DNA damaging treatments would increase the frequency of recombination, as measured by crossovers between allelic markers. Stimulation of allelic recombination was reported in the fruitfly *Drosophila melanogaster* in response to exposure to the DNA damaging agents UV light (Prudhommeau & Proust, 1973), X-rays (Suzuki & Parry, 1964), and mitomycin C (Schewe et al., 1971). X-rays induce recombination in meiotic cells not only of *D. melanogaster* females, but also of males, which normally display no

Increased meiotic recombination in response to X-irradiation has also been reported in

**4. During mitosis and meiosis, DNA damages caused by diverse exogenous**

Molecular recombination (that is homologous physical exchange or informational exchange) during mitosis and meiosis functions as a DNA repair process designated homologous recombinational repair (HRR). Many of the gene products employed in mitotic HRR are also employed in recombination during meiosis. It is this consistent function of recombination across meiosis and mitosis in eukaryotes and transformation in prokaryotes that we seek to understand through the repair hypothesis. Mutants defective in HRR genes in *D. melanogaster* and yeast have reduced ability to repair DNA damages arising from a variety of exogenous sources. These mutants are also defective in recombination during meiosis. In general, loss of HRR capability causes increased sensitivity to killing by agents that harm cells primarily through induction of DNA damage. These agents are listed in Table 1. There have been no reports, that we know of, that HRR defective cells are sensitive to agents that

In *D. melanogaster*, mutants defective in genes *mei-41, mei-9, hdm, spnA* and *brca2* have reduced spontaneous allelic recombination (crossing over) during meiosis and increased sensitivity to killing by exposure to numerous DNA damaging agents (Table 1). The Mei-41 protein is a structural and functional homolog of the human Atm (ataxia telangiectasia) protein (Hari et al., 1995), which plays a central role in HRR. The Mei-9 and Hdm proteins are components of a multiprotein complex that resolves meiotic recombination intermediates (Joyce et al., 2009). The SpnA protein is a homolog of yeast Rad51 (Staeva-Vieira et al., 2003), and Rad51 plays a central role in strand-exchange during HRR. The *D. melanogaster* Brca2 protein, a homolog of the human Brca2 protein that protects against breast cancer, regulates the activity of Rad51 protein in HRR. The Brca2 protein is required

*Caenorhabditis elegans* (Kim & Rose, 1987), and in *S. cerevisiae* (Kelly et al., 1983).

**agents can be repaired by homologous recombination**

harm cells by mechanisms other than primarily causing DNA damage.

for HRR of DSBs during meiosis (Klovstad et al., 2008).

particularly oxidative DNA damage.

recombination during meiosis (Hannah-Alava, 1964).

**recombination**

evolutionary origin and was present in the last common ancestor of eukaryotes. Recently, this view received further support from a study of amoebae. Although amoebae generally have been assumed to be asexual, Lahr et al. (2011) showed that the majority of amoeboid lineages were likely anciently sexual, and that most asexual groups have probably arisen recently and independently.

Eukaryotes arose in evolution from prokaryotes, and eukaryotic meiosis may have arisen from bacterial transformation, a naturally occurring sexual process in prokaryotes. The fundamental similarities between transformation and meiosis have been explored (H. Bernstein & C. Bernstein, 2010). Bacterial transformation, like meiosis, involves alignment and recombination between non-sister homologous chromosomes (or parts of chromosomes) originating from different parents. Both during transformation and meiosis, homologs of the bacterial *recA* gene play a central role in the strand transfer reactions of recombination, indicating a mechanistic similarity. Also, bacterial transformation is induced by environmental stresses that are similar to those that induce meiosis in protists and simple multicellular eukaryotes, suggesting that there was continuity in the evolutionary transition from prokaryotic sex to eukaryotic sex. Evidence indicates that bacterial transformation is an adaptation for repairing DNA (Michod et al., 1988; Hoelzer & Michod, 1991; Michod & Wojciechowski, 1994; reviewed by Michod et al., 2008). Thus meiosis may have emerged from transformation as an adaptation for repairing DNA.

 Meiosis and sex in these organisms is usually induced by stressful conditions. The paramecium tetrahymena can be induced to undergo conjugation leading to meiosis by washing, which causes rapid starvation (Elliott & Hayes, 1953). Depletion of the nitrogen source in the growth medium of the unicellular green alga *Chlamydomonas reinhardi* leads to differentiation of vegetative cells into gametes (Sager & Granick 1954). These gametes can then mate, form zygotes and undergo meiosis. Upon nitrogen starvation or desiccation, the human fungal pathogen *Cryptococcus neoformans* undergoes mating or fruiting, both processes involving meiosis (Lin et al., 2005).

In addition to starvation, oxidative stress is another condition that induces meiosis and sex. The haploid fission yeast *Schizosaccharomyces pombe* is induced to undergo sexual development and mating when the supply of nutrients becomes limiting (Davey et al., 1998). Moreover, treatment of late-exponential-phase *S. pombe* vegetative cells with hydrogen peroxide, which causes oxidative stress, increases the frequency of mating and production of meiotic spores by 4- to 18-fold (C. Bernstein & Johns, 1989). The oomycete *Phytophthora cinnamomi* is induced to undergo sexual reproduction by exposure to the oxidizing agent hydrogen peroxide or mechanical damage to hyphae (Reeves & Jackson, 1974). In the simple multicellular green algae *Volvox carteri,* sex is induced by heat shock (Kirk & Kirk, 1986). This effect can be inhibited by antioxidants, indicating that the induction of sex by heat shock is mediated by oxidative stress (Nedelcu & Michod, 2003). Furthermore, induction of oxidative stress by an inhibitor of the mitochondrial electron transport chain also induced sex in *V. carteri* (Nedelcu et al., 2004). The budding yeast *Saccharomyces cerevisiae* reproduces as mitotically dividing diploid cells when nutrients are plentiful, but undergoes meiosis to form haploid spores when starved (Herskowitz, 1988). When *S. cerevisiae* are starved, oxidative stress is increased and DNA double-strand breaks (DSBs) and apurinic/apyrimidinic sites accumulate (Steinboeck et al., 2010). Perhaps, in *S. cerevisiae,* the induction of sex by starvation is mediated by oxidative stress, analogous to the way induction of sex by heat is mediated by oxidative stress in *V. carteri*.

evolutionary origin and was present in the last common ancestor of eukaryotes. Recently, this view received further support from a study of amoebae. Although amoebae generally have been assumed to be asexual, Lahr et al. (2011) showed that the majority of amoeboid lineages were likely anciently sexual, and that most asexual groups have probably arisen

Eukaryotes arose in evolution from prokaryotes, and eukaryotic meiosis may have arisen from bacterial transformation, a naturally occurring sexual process in prokaryotes. The fundamental similarities between transformation and meiosis have been explored (H. Bernstein & C. Bernstein, 2010). Bacterial transformation, like meiosis, involves alignment and recombination between non-sister homologous chromosomes (or parts of chromosomes) originating from different parents. Both during transformation and meiosis, homologs of the bacterial *recA* gene play a central role in the strand transfer reactions of recombination, indicating a mechanistic similarity. Also, bacterial transformation is induced by environmental stresses that are similar to those that induce meiosis in protists and simple multicellular eukaryotes, suggesting that there was continuity in the evolutionary transition from prokaryotic sex to eukaryotic sex. Evidence indicates that bacterial transformation is an adaptation for repairing DNA (Michod et al., 1988; Hoelzer & Michod, 1991; Michod & Wojciechowski, 1994; reviewed by Michod et al., 2008).  Meiosis and sex in these organisms is usually induced by stressful conditions. The paramecium tetrahymena can be induced to undergo conjugation leading to meiosis by washing, which causes rapid starvation (Elliott & Hayes, 1953). Depletion of the nitrogen source in the growth medium of the unicellular green alga *Chlamydomonas reinhardi* leads to differentiation of vegetative cells into gametes (Sager & Granick 1954). These gametes can then mate, form zygotes and undergo meiosis. Upon nitrogen starvation or desiccation, the human fungal pathogen *Cryptococcus neoformans* undergoes mating or fruiting, both

In addition to starvation, oxidative stress is another condition that induces meiosis and sex. The haploid fission yeast *Schizosaccharomyces pombe* is induced to undergo sexual development and mating when the supply of nutrients becomes limiting (Davey et al., 1998). Moreover, treatment of late-exponential-phase *S. pombe* vegetative cells with hydrogen peroxide, which causes oxidative stress, increases the frequency of mating and production of meiotic spores by 4- to 18-fold (C. Bernstein & Johns, 1989). The oomycete *Phytophthora cinnamomi* is induced to undergo sexual reproduction by exposure to the oxidizing agent hydrogen peroxide or mechanical damage to hyphae (Reeves & Jackson, 1974). In the simple multicellular green algae *Volvox carteri,* sex is induced by heat shock (Kirk & Kirk, 1986). This effect can be inhibited by antioxidants, indicating that the induction of sex by heat shock is mediated by oxidative stress (Nedelcu & Michod, 2003). Furthermore, induction of oxidative stress by an inhibitor of the mitochondrial electron transport chain also induced sex in *V. carteri* (Nedelcu et al., 2004). The budding yeast *Saccharomyces cerevisiae* reproduces as mitotically dividing diploid cells when nutrients are plentiful, but undergoes meiosis to form haploid spores when starved (Herskowitz, 1988). When *S. cerevisiae* are starved, oxidative stress is increased and DNA double-strand breaks (DSBs) and apurinic/apyrimidinic sites accumulate (Steinboeck et al., 2010). Perhaps, in *S. cerevisiae,* the induction of sex by starvation is mediated by oxidative stress, analogous to the

way induction of sex by heat is mediated by oxidative stress in *V. carteri*.

recently and independently.

from transformation as an adaptation for repairing DNA.

processes involving meiosis (Lin et al., 2005).

These observations suggest that meiosis is an adaptation for dealing with stress, particularly oxidative stress. It is well established that oxidative stress induces a variety of DNA damages including DNA DSBs, single-strand breaks and modified bases (Slupphaug et al., 2003). Thus we hypothesize that, in facultative sexual protists and simple multicellular eukaryotes, sex, with the central feature of meiosis, is an adaptive response to DNA damage, particularly oxidative DNA damage.