DEVELOPMENTAL SUCCESS, STABILITY, AND PLASTICITY IN CLOSELY RELATED PARTHENOGENETIC AND SEXUAL LIZARDS (HETERONOTIA, GEKKONIDAE)

Premeiotic endoreplication is essential for obligate parthenogenesis in geckos

Obligate parthenogenesis evolved in reptiles convergently several times, mainly through interspecific hybridization. The obligate parthenogenetic complexes typically include both diploid and triploid lineages. Offspring of parthenogenetic hybrids are genetic copies of their mother; however, the cellular mechanism enabling the production of unreduced cells is largely unknown. Here, we show that oocytes go through meiosis in three widespread, or even strongly invasive, obligate parthenogenetic complexes of geckos, namely in diploid and triploid Lepidodactylus lugubris, and triploid Hemiphyllodactylus typus and Heteronotia binoei. In all four lineages, the majority of oocytes enter the pachytene at the original ploidy level, but their chromosomes cannot pair properly and instead form univalents, bivalents and multivalents. Unreduced eggs with clonally inherited genomes are formed from germ cells that had undergone premeiotic endoreplication, in which appropriate segregation is ensured by the formation of bivalents made from copies of identical chromosomes. We conclude that the induction of premeiotic endoreplication in reptiles was independently co-opted at least four times as an essential component of parthenogenetic reproduction and that this mechanism enables the emergence of fertile polyploid lineages within parthenogenetic complexes.

Heightened among-individual variation in life history but not morphology is related to developmental temperature in reptiles

Increases in phenotypic variation under extreme (e.g. novel or stressful) environmental conditions are emerging as a crucial process through which evolutionary adaptation can occur. Lack of prior stabilizing selection, as well as potential instability of developmental processes in these environments, may lead to a release of phenotypic variation that can have important evolutionary consequences. Although such patterns have been shown in model study organisms, we know little about the generality of trait variance across environments for non-model organisms. Here, we test whether extreme developmental temperatures increase the phenotypic variation across diverse reptile taxa. We find that the among-individual variation in a key life-history trait (post-hatching growth) increases at extreme cold and hot temperatures. However, variations in two measures of hatchling morphology and in hatchling performance were not related to developmental temperature. Although extreme developmental temperatures may increase the variation in growth, our results suggest that plastic responses to stressful incubation conditions do not generally make more extreme phenotypes available to selection. We discuss the reasons for the general lack of increased variability at extreme incubation temperatures and the implications this has for local adaptation in hatchling morphology and physiology.

Evolutionary Dynamics and Consequences of Parthenogenesis in Vertebrates

Parthenogenesis is asexual reproduction without any required participation from males and, as such, is a null model for sexual reproduction. In a comparative context, we can expand our understanding of the evolution and ecology of sex by investigating the consequences of parthenogenesis. In this review, we examine the theoretical predictions of and empirical results on the evolution of asexual reproduction in vertebrates, focusing on recent studies addressing the origins and geographic spread of parthenogenetic lineages and the genomic consequences of an asexual life history. With advances in computational methods and genome technologies, researchers are poised to make rapid and significant progress in studying the origin and evolution of parthenogenesis in vertebrates, thus providing an important perspective on understanding biodiversity patterns of both asexual and sexual populations.

Thermal variability during ectotherm egg incubation: A synthesis and framework

Natural populations of ectothermic oviparous vertebrates typically experience thermal variability in their incubation environment. Yet an overwhelming number of laboratory studies incubate animals under constant thermal conditions that cannot capture natural thermal variability. Here, we systematically searched for studies that incubated eggs of ectothermic vertebrates, including both fishes and herpetofauna, under thermally variable regimes. We ultimately developed a compendium of 66 studies that used thermally variable conditions for egg incubation. In this review, we qualitatively discuss key findings from literature in the compendium, including the phenotypic effects resulting from different patterns of thermally variable incubation, as well as the ontogenetic persistence of these effects. We also describe a physiological framework for contextualizing some of these effects, based on thermal performance theory. Lastly, we highlight key gaps in our understanding of thermally variable incubation and offer suggestions for future studies.

Comparing developmental stability in unisexual and bisexual rock lizards of the genus Darevskia

Parthenogenetic species are usually considered to be short-lived due to the accumulation of adverse mutations, lack of genetic variability, and inability to adapt to changing environment. If so, one may expect that the phenotype of clonal organisms may reflect such genetic and/or environmental stress. To test this hypothesis, we compared the developmental stability of bisexual and parthenogenetic lizards of the genus Darevskia. We assessed asymmetries in three meristic traits: ventral, preanal, and supratemporal scales. Our results suggest that the amount of ventral and preanal asymmetries is significantly higher in clones compared with their maternal, but not paternal, progenitor species. However, it is questionable, whether this is a consequence of clonality, as it may be considered a mild form of outbreeding depression as well. Moreover, most ventral asymmetries were found in the bisexual species Darevskia valentini. We suggest that greater differences in asymmetry levels among bisexuals may be, for instance, a consequence of the population size: the smaller the population, the higher the inbreeding and the developmental instability. On the basis of the traits examined in this study, the parthenogens do not seem to be of significantly poorer quality.

Is meiosis a fundamental cause of inviability among sexual and asexual plants and animals?

Differences in viability between asexually and sexually generated offspring strongly influence the selective advantage and therefore the prevalence of sexual reproduction (sex). However, no general principle predicts when sexual offspring will be more viable than asexual offspring. We hypothesize that when any kind of reproduction is based on a more complex cellular process, it will encompass more potential failure points, and therefore lower offspring viability. Asexual reproduction (asex) can be simpler than sex, when offspring are generated using only mitosis. However, when asex includes meiosis and meiotic restitution, gamete production is more complex than in sex. We test our hypothesis by comparing the viability of asexual and closely related sexual offspring across a wide range of plants and animals, and demonstrate that meiotic asex does result in lower viability than sex; without meiosis, asex is mechanistically simple and provides higher viability than sex. This phylogenetically robust pattern is supported in 42 of 44 comparisons drawn from diverse plants and animals, and is not explained by the other variables included in our model. Other mechanisms may impact viability, such as effects of reproductive mode on heterozygosity and subsequent viability, but we propose the complexity of cellular processes of reproduction, particularly meiosis, as a fundamental cause of early developmental failure and mortality. Meiosis, the leading cause of inviability in humans, emerges as a likely explanation of offspring inviability among diverse eukaryotes.

Mitochondrial DNA suggests a single maternal origin for the widespread triploid parthenogenetic pest species, Paratanytarsus grimmii, but microsatellite variation shows local endemism

Parthenogenesis is common among invasive pest species, with many parthenogenetic species also showing polyploidy. Parthenogenetic polyploid species often have multiple hybrid origins and the potential to rapidly spread over vast geographical areas. In this study, we examine patterns of mitochondrial and microsatellite variation in a widespread triploid parthenogenetic chironomid pest species, Paratanytarsus grimmii. Based on samples from five countries, including Australia, England, Germany, Japan, and Canada, we found extremely low mitochondrial diversity (<0.14%), with most individuals sharing a common and widespread haplotype. In contrast, microsatellite diversity revealed 41 clonal variants, which were regionally endemic. These findings suggest a single invasive maternal lineage of P. grimmii is likely to have recently spread over a broad geographical range. High levels of genotypic endemism suggest P. grimmii populations have remained relatively isolated after an initial spread, with little ongoing migration. This, in part, can be attributed to rapid genetic differentiation via mutations of common clonal genotypes after P. grimmii spread, but multiple polyploidization and subsequent founder events are also likely to be contributing factors.

Does incubation temperature fluctuation influence hatchling phenotypes in reptiles? A test using parthenogenetic geckos

Many lineages of parthenogenetic organisms have persisted through significant environmental change despite the constraints imposed by their fixed genotype and limited evolutionary potential. The ability of parthenogens to occur sympatrically with sexual relatives may in part be due to phenotypic plasticity in their responses to their environment, especially with respect to incubation temperature--a maternally selected trait. Here we measured the incubation temperatures selected by two lineages of triploid parthenogenic geckos in the Heteronotia binoei complex by allowing them to deposit clutches along a thermal gradient. The average nest temperature selected was 28.4 degrees C, with no significant differences between parthenogenic races or individual clones. To investigate the effect of nest-temperature variability on physiological and morphological traits, we incubated eggs from different races at one of four incubation regimes (32 degrees +/- 0 degrees, +/- 3 degrees , +/- 5 degrees , or +/- 9 degrees C). Embryos incubated at constant 32 degrees C developed faster than embryos reared under increasing extremes of diel temperature fluctuation (+/- 3 degrees , +/- 5 degrees C), and incubation at 32 degrees +/- 9 degrees C was unsuccessful. Incubation regime had no effect on the body size, preferred substrate temperature, or mass-specific .V(O2) of hatchlings. However, parthenogenic race had a significant effect on egg mass, tail length, snout-to-vent length, total length, and .V(O2) . We conclude that developmental traits are strongly influenced by clonal genotypes in this parthenogenic complex but are well buffered against fluctuations in incubation temperature.

Diversification and persistence at the arid-monsoonal interface: australia-wide biogeography of the Bynoe's gecko (Heteronotia binoei; Gekkonidae)

Late Neogene aridification in the Southern Hemisphere caused contractions of mesic biota to refugia, similar to the patterns established by glaciation in the Northern Hemisphere, but these episodes also opened up new adaptive zones that spurred range expansion and diversification in arid-adapted lineages. To understand these dynamics, we present a multilocus (nine nuclear introns, one mitochondrial gene) phylogeographic analysis of the Bynoe's gecko (Heteronotia binoei), a widely distributed complex spanning the tropical monsoon, coastal woodland, and arid zone biomes in Australia. Bayesian phylogenetic analyses, estimates of divergence times, and demographic inferences revealed episodes of diversification in the Pliocene, especially in the tropical monsoon biome, and range expansions in the Pleistocene. Ancestral habitat reconstructions strongly support recent and independent invasions into the arid zone. Our study demonstrates the varied responses to aridification in Australia, including localized persistence of lineages in the tropical monsoonal biome, and repeated invasion of and expansion through newly available arid-zone habitats. These patterns are consistent with those found in other arid environments in the Southern Hemisphere, including the South African succulent karoo and the Chilean lowlands, and highlight the diverse modes of diversification and persistence of Earth's biota during the glacial cycles of the Pliocene and Pleistocene.

Worldwide invasion by the little fire ant: routes of introduction and eco-evolutionary pathways

Biological invasions are generally thought to occur after human aided migration to a new range. However, human activities prior to migration may also play a role. We studied here the evolutionary genetics of introduced populations of the invasive ant Wasmannia auropunctata at a worldwide scale. Using microsatellite markers, we reconstructed the main routes of introduction of the species. We found three main routes of introduction, each of them strongly associated to human history and trading routes. We also demonstrate the overwhelming occurrence of male and female clonality in introduced populations of W. auropunctata, and suggest that this particular reproduction system is under selection in human-modified habitats. Together with previous researches focused on native populations, our results suggest that invasive clonal populations may have evolved within human modified habitats in the native range, and spread further from there. The evolutionarily most parsimonious scenario for the emergence of invasive populations of the little fire ant might thus be a two-step process. The W. auropunctata case illustrates the central role of humans in biological change, not only due to changes in migration patterns, but also in selective pressures over species.

Thelytokous parthenogenesis, male clonality and genetic caste determination in the little fire ant: new evidence and insights from the lab

Previous studies indicate that some populations of the little fire ant, Wasmannia auropunctata, display an unusual reproduction system polymorphism. Although some populations have a classical haplodiploid reproduction system, in other populations queens are produced by thelytokous parthenogenesis, males are produced by a male clonality system and workers are produced sexually. An atypical genetic caste determination system was also suggested. However, these conclusions were indirectly inferred from genetic studies on field population samples. Here we set up experimental laboratory nests that allow the control of the parental relationships between individuals. The queens heading those nests originated from either putatively clonal or sexual populations. We characterized the male, queen and worker offspring they produced at 12 microsatellite loci. Our results unambiguously confirm the unique reproduction system polymorphism mentioned above and that male clonality is strictly associated with thelytokous parthenogenesis. We also observed direct evidence of the rare production of sexual gynes and arrhenotokous males in clonal populations. Finally, we obtained evidence of a genetic basis for caste determination. The evolutionary significance of the reproduction system polymorphism and genetic caste determination as well as future research opportunities are discussed.

Reproductive system, social organization, human disturbance and ecological dominance in native populations of the little fire ant, Wasmannia auropunctata

The invasive ant species Wasmannia auropunctata displays both ecologically dominant and non-dominant populations within its native range. Three factors could theoretically explain the ecological dominance of some native populations of W. auropunctata: (i) its clonal reproductive system, through demographic and/or adaptive advantages; (ii) its unicolonial social organization, through lower intraspecific and efficient interspecific competition; (iii) the human disturbance of its native range, through the modification of biotic and abiotic environmental conditions. We used microsatellite markers and behavioural tests to uncover the reproductive modes and social organization of dominant and non-dominant native populations in natural and human-modified habitats. Microsatellite and mtDNA data indicated that dominant and non-dominant native populations (supercolonies as determined by aggression tests) of W. auropunctata did not belong to different evolutionary units. We found that the reproductive system and the social organization are neither necessary nor sufficient to explain W. auropunctata ecological dominance. Dominance rather seems to be set off by unknown ecological factors altered by human activities, as all dominant populations were recorded in human-modified habitats. The clonal reproductive system found in some populations of W. auropunctata may however indirectly contribute to its ecological dominance by allowing the species to expand its environmental niche, through the fixation over time of specific combinations of divergent male and female genotypes. Unicoloniality may rather promote the range expansion of already dominant populations than actually trigger ecological dominance. The W. auropunctata model illustrates the strong impact of human disturbance on species' ecological features and the adaptive potential of clonal reproductive systems.

Effects of temperature on embryonic development of the veiled chameleon, Chamaeleo calyptratus

Temperature dependence of development of the chameleon, Chamaeleo calyptratus, was assessed from observations on eggs incubated at 25, 28 and 30 degrees C. Overall, differentiation, growth in mass, and growth of the yolk sac and chorioallantois were the slowest at 25 degrees C but did not differ between 28 and 30 degrees C. The relative area of the yolk sac (YS), chorioallantoic membrane (CAM), and their precursor, the area opaca vasculosa (AV) was used to characterize developmental phases. During Phase 1, only the AV was present; development was characterized by differentiation with little increase in the size of the embryo. During Phase 2, the vascularized YS and CAM grew from about 10 to 100% coverage of the surface of the shell during a period of about two weeks. Differentiation and growth of the embryo were accordingly rapid. During Phase 3, the YS and CAM were fixed in size and the remainder of development was relatively slow. Characterization of embryonic development with respect to the relative area of the AV-YS-CAM highlighted the functional linkage between development and the systems that provide nutrients to embryos.

Error propagation across levels of organization: from chemical stability of ribosomal RNA to developmental stability

My hypothesis integrates molecular and whole-organism levels of development. A physico-chemical property of nucleotides (their dipole moment), confers structural thermostability on double-stranded sequences, and decreases chemical stability of single-stranded sequences. According to this approach, low ribosomal RNA stability should decrease the precision of protein synthesis and whole-organism developmental stability. Indeed, substitution frequencies in pseudogenes are proportional to the subtraction of the dipole moment of the substituting nucleotide from that of the substituted one, and developmental instability, estimated by morphological fluctuating asymmetry (FA), correlates with mammal 12s rRNA base content of loop (but not stem) regions. In lizards, fit to the single-strand rationale of sequence chemical stability decreases with the level of poikilothermy of the investigated lizard family, suggesting interactions between changes in body temperature, ribosomal structure and developmental instability. Results confirm the hypothesis (less than for 12s rRNA) in: third codon positions of cytochrome B, probably because, unlike rRNAs, specific mRNAs affect only the protein they code; and 16s rRNA, apparently because its base composition is more affected by genome-wide mutational biases than that of 12s rRNA.

Phylogeography of sexual Heteronotia binoei (Gekkonidae) in the Australian arid zone: climatic cycling and repetitive hybridization

The biota of much of continental Australia have evolved within the context of gradual aridification of the region over several million years, and more recently of climatic cycling between relatively dry and humid conditions. We performed a phylogeographical study of three sexual chromosome races of the Heteronotia binoei complex of geckos found throughout the Australian arid zone. Two of these three races were involved in two separate hybridization events leading to parthenogenetic lineages (also H. binoei), and the third is widespread and broadly sympatric with the parthenogens. Based on our analyses, the three sexual races diversified approximately 6 million years ago in eastern Australia, during a period of aridification, then each moved west through northern, southern, and central dispersal corridors to occupy their current ranges. In each case, the timing of major phylogeographical inferences corresponds to inferred palaeoclimatic changes in continental Australia. This scenario provides a simple explanation for diversification, secondary contact, and hybridization between the races. However, data presented elsewhere indicate that formation of the parthenogens was considerably more recent than the westward expansion of the hybridizing races, and that multiple hybridization events were geographically and temporally distinct. We suggest that cyclical climate changes may have led to regional range changes that facilitated hybridization between the races, which are not currently known to be in sympatry.

Lower fecundity in parthenogenetic geckos than sexual relatives in the Australian arid zone

Theoretical models of the advantage of sexual reproduction typically assume that reproductive output is equal in sexual and parthenogenetic females. We tested this assumption by comparing fecundity between parthenogenetic and sexual races of gekkonid lizards in the Heteronotia binoei complex, collected across a 1200 km latitudinal gradient through the Australian arid zone. Under laboratory conditions, parthenogenetic geckos had approximately 30% lower fecundity when compared with their sexual progenitors, irrespective of body size. Reflecting clutch-size constraints in gekkonids, this fecundity difference was mainly because of fewer clutches over a shorter period. When parthenogens were compared more broadly with all coexisting sexual races across the latitudinal gradient, parthenogens had lower fecundity than sexuals only when corrected for body size. Differences in fecundity between parthenogens and coexisting sexual races depended on which sexual race was considered. There was no significant relationship between fecundity and parasite (mite) load, despite significantly higher mite loads in parthenogens than in sexual races.