Evolutionary Dynamics of Mitochondrial DNA Duplications in Parthenogenetic Geckos, Heteronotia Binoei

Mitochondrial DNA (mtDNA) from triploid parthenogenetic geckos of the Heteronotia binoei complex varies in size from 17.2 to 27.6 kilobases (kb). Comparisons of long vs. short genomes using restriction endonucleases revealed a series of tandem direct duplications ranging in size from 1.2 to 10.4 kb. This interpretation was supported by transfer-hybridization experiments which also demonstrated that coding sequences were involved. Some of the duplications have been modified by deletion and restriction site changes, but no other rearrangements were detected. Analysis of the phylogenetic and geographic distribution of length variation suggests that duplications have arisen repeatedly within the parthenogenetic form of H. binoei. The parthenogens, and thus the duplications, are of recent origin; modifications of the duplicated sequences, particularly by deletion, has therefore been rapid. The absence of duplications from the mtDNA of the diploid sexual populations of H. binoei reinforces the correlation between nuclear polyploidy and duplication of mtDNA sequences reported for other lizards. In comparison to the genomes of sexual H. binoei and of most other animals, the mtDNA of these parthenogenetic geckos is extraordinarily variable in length and organization.     

Nonadaptive evolution of mitochondrial genome size

Genomes vary greatly in size and complexity, and identifying the evolutionary forces that have generated this variation remains a major goal in biology. A controversial proposal is that most changes in genome size are initially deleterious and therefore are linked to episodes of decrease in effective population sizes. Support for this hypothesis comes from large-scale comparative analyses, but vanishes when phylogenetic nonindependence is taken into account. Another approach to test this hypothesis involves analyzing sequence evolution among clades where duplications have recently fixed. Here we show that episodes of fixation of duplications in mitochondrial genomes of the gecko Heteronotia binoei (two independent clades) and of mantellid frogs (five distinct branches) coincide with reductions in the ability of selection to purge slightly deleterious mutations. Our results support the idea that genome complexity can arise through nonadaptive processes in tetrapods.     

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.     

The ecology of the Australian gecko Heteronotia binoei in northern New South Wales

A two year study was carried out on Heteronotia binoei (Gray) in the Pilliga Scrub by means of visits of six days' duration in each month of the year. Animals were individually marked by toe-clipping and a mark-recapture programme was carried out throughout the study.
In the Pilliga Scrub Heteronotia lives below bark or boards on the ground or below the bark at the base of dead trees or stumps.
The mark-recapture programme provided extensive data on the population, its movements, growth, tail loss and regeneration. Samples were collected for analysis of intestinal contents. The above data together with observations of behaviour provided a well corroborated picture of the biology of this small gekkonid lizard.     

Ovarian development in sexual and parthenogenetic geckos of the Heteronotia binoei complex

Ovarian development in sexual and parthenogenetic geckos of the Heteronotia binoei complex was analysed quantitatively by gross and histological examination. Females were classified into one of four stages of the reproductive cycle: non-reproductive, preovulatory, postovulatory and postoviposition. Parthenogens had significantly more developing follicles present in the ovaries than sexual females in non-reproductive, preovulatory and postovulatory stages of the reproductive cycle. The larger number of developing follicles in parthenogenetic Heteronotia was correlated with the significantly larger body size of the parthenogenetic females at the localities examined. Maximum follicular size and rates of follicular atresia were not significantly different between sexual and parthenogenetic females. These findings raise the possibility that the reproductive output of the parthenogenetic females may be higher than that of sympatric sexual females at these localities due to an increase in follicular recruitment. Based on this evidence, we predict that at these localities parthenogenetic females produce more clutches of two eggs than sexual females. If viability is equal, this would enhance the reproductive advantage of parthenogens over the two-fold level already present. In all other respects, the morphology of the ovaries appeared very similar between parthenogenetic and sexual females and was typical of geckos.     

The Effect of Chemical Information on the Spatial Distribution of Fruit Flies: I Model Results

Animal aggregation is a general phenomenon in ecological systems. Aggregations are generally considered as an evolutionary advantageous state in which members derive the benefits of protection and mate choice, balanced by the costs of limiting resources and competition. In insects, chemical information conveyance plays an important role in finding conspecifics and forming aggregations. In this study, we describe a spatio-temporal simulation model designed to explore and quantify the effects of these infochemicals, i.e., food odors and an aggregation pheromone, on the spatial distribution of a fruit fly (Drosophila melanogaster) population, where the lower and upper limit of local population size are controlled by an Allee effect and competition. We found that during the spatial expansion and strong growth of the population, the use of infochemicals had a positive effect on population size. The positive effects of reduced mortality at low population numbers outweighed the negative effects of increased mortality due to competition. At low resource densities, attraction toward infochemicals also had a positive effect on population size during recolonization of an area after a local population crash, by decreasing the mortality due to the Allee effect. However, when the whole area was colonized and the population was large, the negative effects of competition on population size were larger than the positive effects of the reduction in mortality due to the Allee effect. The use of infochemicals thus has mainly positive effects on population size and population persistence when the population is small and during the colonization of an area. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

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.     

A neutral model with fluctuating population size and its effective size

We consider a diffusion model with neutral alleles whose population size is fluctuating randomly. For this model, the effects of fluctuation of population size on the effective size are investigated. The effective size defined by the equilibrium average heterozygosity is larger than the harmonic mean of population size but smaller than the arithmetic mean of population size. To see explicitly the effects of fluctuation of population size on the effective size, we investigate a special case where population size fluctuates between two distinct states. In some cases, the effective size is very different from the harmonic mean. For this concrete model, we also obtain the stationary distribution of the average heterozygosity. Asymptotic behavior of the effective size is obtained when the population size is large and/or autocorrelation of the fluctuation is weak or strong.     

Parental environmental effects and cyclical dynamics in plant populations

Abstract Parental environmental effects have been widely reported in plants, but these effects are often weak relative to direct effects of current environmental conditions. Few studies have asked when consideration of such effects is necessary to understand long-term plant population dynamics. In this article, I show that inclusion of effects of parental density on offspring mass fundamentally changes population dynamics models by making recruitment a function of population size in two previous generations ([Formula: see text]), rather than one ([Formula: see text]). Models without parental density effects predict either stable population dynamics or sharp crashes from high to low population size (flip bifurcations). When parental effects are at least one-third the size of direct density effects, gradual cycles from high to low population size (Hopf bifurcations) are possible. In this study, I measured effects of parental and offspring density on offspring quality in an annual plant, Cardamine pensylvanica, by manipulating plant density independently in parent and offspring generations and by comparing the effects of parent and offspring density on offspring performance. Parental density effects were detectable but were noticeably weaker than offspring density effects. Nonetheless, the parental effect was large enough to change population dynamics predictions. Thus, parental effects may be an important component of the numerical dynamics of plant populations.     

Relation of population changes to the distribution of genetic factors

Abstract

During the 1800's, the population of Ireland underwent a rapid increase and subsequent decrease in population size. The effects of this change upon population structure were assessed using a simulation of the isolation by distance model and comparing the results to those obtained assuming constant population size. These results indicate that changes in within‐group genetic similarity (kinship) brought about by a rapid increase in population size are cancelled by the effects of a rapid decrease in population size. Parameters of the isolation by distance model are hardly affected by population size changes. These results suggest that violation of the assumption of constant population size for population structure models may not be that serious when population size changes rapidly and in both directions.     

Propagule pressure and persistence in experimental populations

Average inoculum size and number of introductions are known to have positive effects on population persistence. However, whether these factors affect persistence independently or interact is unknown. We conducted a two-factor experiment in which 112 populations of parthenogenetic Daphnia magna were maintained for 41 days to study effects of inoculum size and introduction frequency on: (i) population growth, (ii) population persistence and (iii) time-to-extinction. We found that the interaction of inoculum size and introduction frequency-the immigration rate-affected all three dependent variables, while population growth was additionally affected by introduction frequency. We conclude that for this system the most important aspect of propagule pressure is immigration rate, with relatively minor additional effects of introduction frequency and negligible effects of inoculum size.     

Genetic diversity, structure, and size of an endangered brown bear population threatened by highway construction in the Pindos Mountains, Greece

One of the major negative effects of roads is the creation of barriers to the movement of wildlife, ultimately disconnecting populations and increasing extinction risk. We collected genetic data from a threatened brown bear population in the central part of the Pindos mountain range in northwestern Greece to provide information about this, as yet genetically undescribed, population and to evaluate its status prior to the construction of a major highway. We used noninvasive genetic sampling methods and microsatellite analysis to investigate nuclear genetic diversity, population genetic structure, demographic history, relatedness within the population and estimated effective and total population size. Brown bears in the study area were found to possess a relatively high level of nuclear genetic diversity and low levels of inbreeding; the population did not show any signs of substructuring but seems to have gone through a genetic bottleneck in the recent past. The estimated effective population size was 29, and the total population size estimate obtained by two different methods was 33 and 51 individuals, respectively. Our results indicate a good conservation status of this bear population and provide baseline genetic data for the future evaluation of the effects on bears from the construction of a major highway, for monitoring the genetic status of this and other bear populations in Greece and for assessing gene flow in bear populations in southern Europe.     

Developmental success, stability, and plasticity in closely related parthenogenetic and sexual lizards (Heteronotia, Gekkonidae)

The developmental trajectory of an organism is influenced by the interaction between its genes and the environment in which it develops. For example, the phenotypic traits of a hatchling reptile can be influenced by the organism's genotype, by incubation temperature, and by genetically coded norms of reaction for thermally labile traits. The evolution of parthenogenesis provides a unique opportunity to explore such effects: a hybrid origin of this trait in vertebrates modifies important aspects of the genotype (e.g., heterozygosity, polyploidy) and may thus impact not only on the phenotype generally, but also on the ways in which incubation temperature affects expression of the phenotype. The scarcity of vertebrate parthenogenesis has been attributed to developmental disruptions, but previous work has rarely considered reaction norms of embryogenesis in this respect. We used closely related sexual and asexual races of the Australian gecko Heteronotia binoei, which include those with multiple origins of parthenogenesis, to explore the ways in which reproductive modes (sexual, asexual), incubation temperatures (24, 27, and 30 degrees C), and the interaction between these factors affected hatchling phenotypes. The hatchling traits we considered included incubation period, incidence of deformities, hatchling survivorship, body size and shape, scalation (including fluctuating asymmetry), locomotor performance, and growth rate. Developmental success was slightly reduced (higher proportion of abnormal offspring) in parthenogenetic lineages although there was no major difference in hatching success. Incubation temperature affected a suite of traits including incubation period, tail length, body mass relative to egg mass, labial scale counts, running speed, growth rate, and hatchling survival. Our data also reveal an interaction between reproductive modes and thermal regimes, with the phenotypic traits of parthenogenetic lizards less sensitive to incubation temperature than was the case for their sexual relatives. Thus, the evolution of asexual reproduction in this species complex has modified both mean hatchling viability and the norms of reaction linking hatchling phenotypes to incubation temperature. Discussions on the reasons why parthenogenetic organisms are scarce in nature should take into account interactive effects such as these; future work could usefully try to tease apart the roles of parthenogenesis, its hybrid origin (and thus effects on ploidy and heterozygosity, etc.), and clonal selection in generating these divergent embryonic responses.     

Reduced fecundity in small populations of the rare plant Gentianopsis ciliate (Gentianaceae)

Habitat destruction is the main cause for the biodiversity crisis. Surviving populations are often fragmented, i.e., small and isolated from each other. Reproduction of plants in small populations is often reduced, and this has been attributed to inbreeding depression, reduced attractiveness for pollinators, and reduced habitat quality in small populations. Here we present data on the effects of fragmentation on the rare, self-compatible perennial herb Gentianopsis ciliata (Gentianaceae), a species with very small and presumably well-dispersed seeds. We studied the relationship between population size, plant size, and the number of flowers produced in 63 populations from 1996-1998. In one of the years, leaf and flower size and the number of seeds produced per fruit was studied in a subset of 25 populations. Plant size, flower size, and the number of seeds per fruit and per plant increased with population size, whereas leaf length and the number of flowers per plant did not. The effects of population size on reproduction and on flower size remained significant if the effects were adjusted for differences in plant size, indicating that they could not be explained by differences in habitat quality. The strongly reduced reproduction in small populations may be due to pollination limitation, while the reduced flower size could indicate genetic effects.     

Allee effects in stochastic populations

The Allee effect, or inverse density dependence at low population sizes, could seriously impact preservation and management of biological populations. The mounting evidence for widespread Allee effects has lately inspired theoretical studies of how Allee effects alter population dynamics. However, the recent mathematical models of Allee effects have been missing another important force prevalent at low population sizes: stochasticity. In this paper, the combination of Allee effects and stochasticity is studied using diffusion processes, a type of general stochastic population model that accommodates both demographic and environmental stochastic fluctuations. Including an Allee effect in a conventional deterministic population model typically produces an unstable equilibrium at a low population size, a critical population level below which extinction is certain. In a stochastic version of such a model, the probability of reaching a lower size a before reaching an upper size b , when considered as a function of initial population size, has an inflection point at the underlying deterministic unstable equilibrium. The inflection point represents a threshold in the probabilistic prospects for the population and is independent of the type of stochastic fluctuations in the model. In particular, models containing demographic noise alone (absent Allee effects) do not display this threshold behavior, even though demographic noise is considered an "extinction vortex". The results in this paper provide a new understanding of the interplay of stochastic and deterministic forces in ecological populations.     

Malathion applied at standard rates reduces fledgling condition and adult male survival in a wild lesser kestrel population

The lesser kestrel Falco naumanni is a globally threatened colonial bird that has suffered a generalized population decline in Europe. Recent studies have suggested that the land-use changes linked to agriculture intensification are the main factors accounting for the population regression. However, there has been little analysis of the role of pesticide applications. In this study, we examined the consequences of a malathion treatment in a wild population of lesser kestrels, specifically its effects on breeding performance, adult survival and population size. We found that the larger the area treated with malathion around the colony, the lower the size and the body condition of the fledglings, although no effects were found on the number of fledged chicks per pair or their sex ratio. Survival of adult males, but not females, was lower in the malathion-treated areas. These results show that organophosphorus pesticides applied at standard rates might disrupt the lesser kestrel population dynamics by reducing their breeding performance and increasing adult mortality in a sex-biased way. The proportional area treated around the colony did not affect colony size in the following year, indicating that an increased adult male mortality was not enough to lead to a detectable population size reduction and/or that the arrival of immigrants could have masked it. Both the direct malathion toxic effects and the expected reduction in prey availability due to fumigation are likely to underlie the observed effects. Hence, keeping non-treated buffer areas around kestrel colony centers should be a strictly applied conservation measure to avoid the observed negative side effects of malathion applications.     

Combining phylogeography with distribution modeling: multiple Pleistocene range expansions in a parthenogenetic gecko from the Australian arid zone

Phylogenetic and geographic evidence suggest that many parthenogenetic organisms have evolved recently and have spread rapidly. These patterns play a critical role in our understanding of the relative merits of sexual versus asexual reproductive modes, yet their interpretation is often hampered by a lack of detail. Here we present a detailed phylogeographic study of a vertebrate parthenogen, the Australian gecko Heteronotia binoei, in combination with statistical and biophysical modeling of its distribution during the last glacial maximum. Parthenogenetic H. binoei occur in the Australian arid zone and have the widest range of any known vertebrate parthenogen. They are broadly sympatric with their sexual counterparts, from which they arose via hybridization. We have applied nested clade phylogeographic, effective migration, and mismatch distribution analyses to mitochondrial DNA (mtDNA) sequences obtained for 319 individuals sampled throughout the known geographic ranges of two parthenogenetic mitochondrial lineages. These analyses provide strong evidence for past range expansion events from west to east across the arid zone, and for continuing eastward range expansion. Parthenogen formation and range expansion events date to the late Pleistocene, with one lineage expanding from the northwest of its present range around 240,000 years ago and the second lineage expanding from the far west around 70,000 years ago. Statistical and biophysical distribution models support these inferences of recent range expansion, with suitable climatic conditions during the last glacial maximum most likely limited to parts of the arid zone north and west of much of the current ranges of these lineages. Combination of phylogeographic analyses and distribution modeling allowed considerably stronger inferences of the history of this complex than either would in isolation, illustrating the power of combining complementary analytical approaches.     

Forest fragmentation and plant reproductive success: a case study in four perennial herbs

We assessed the effects of habitat fragmentation on reproductive success in natural populations of four forest herbs with differing life-history traits and whose distribution patterns appeared to be negatively affected by decreased habitat size and/or increased isolation: Carex sylvatica, Galium odoratum, Sanicula europaea and Veronica montana. Our aims were to test (1) whether habitat size and isolation are positively correlated with population size and isolation, respectively, (2) whether plant reproductive success, a major component of plant fitness, is reduced in small and/or isolated populations when also accounting for differences in habitat quality (edaphic conditions, light intensity) and the effects of plant size, and (3) whether species with different life histories are affected differently. There were significant positive relationships between habitat and population size and between habitat and population isolation in some, but not all of the species. We mostly found no negative effects of small population size or isolation on reproduction. However, reproductive success was reduced in small populations of Sanicula, and this effect was independent of differences in plant size and environmental conditions. The reduced fecundity in small populations may be a consequence of the Allee-effect, a possible mechanism being pollen limitation. Furthermore, the proportion of flowering ramets was reduced in small and isolated populations of Galium, which may have been caused by changes in population structure. Lastly, we found some evidence for largely outcrossing, non-clonal species to be more sensitive to reductions in population size, at least in terms of their reproductive success.     

The importance of control populations for the identification and management of genetic diversity

A fundamental criterion for recognizing species or populations as potentially endangered is the presence/absence of genetic diversity. However, the lack of control populations in many studies of natural systems deprives one from unambiguous criteria for evaluating the genetic effects of small population size and its potential effects on fitness. In this study, I present an example of how the lack of adequate controls may lead to erroneous conclusions for understanding the role that population size may play in the preservation of genetic diversity and fitness of natural populations. The genetic analysis of a population of greater prairie chickens from Illinois, USA, between two time periods (1974–1987 and 1988–1993) in which the studied population experienced a substantial reduction in size and fitness showed no apparent associations between population size and genetic diversity. However, genetic analysis of museum specimens from early this century indicated that Illinois prairie chickens had originally higher levels of genetic diversity, which suggest the Illinois population was already bottlenecked by the 1970s. This study emphasizes the importance of using historical controls to evaluate the temporal dynamics of genetic variability in natural populations. The large number of museum collections worldwide may provide a valuable source of genetic information from past populations, particularly in species currently endangered as a result of human activities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Geographical variation in offspring size effects across generations

Offspring size is thought to strongly affect offspring fitness and many studies have shown strong offspring size/fitness relationships in marine and terrestrial organisms. This relationship is strongly mitigated by local environmental conditions and the optimal offspring size that mothers should produce will vary among different environments. It is assumed that offspring size will consistently affect the same traits among populations but this assumption has not been tested. Here I use a common garden experiment to examine the effects of offspring size on subsequent performance for the marine bryozoan Bugula neritina using larvae from two very different populations. The local conditions at one population (Williamstown) favour early reproduction whereas the other population (Pt. Wilson) favours early growth. Despite being placed in the same habitat, the effects of parental larval size were extremely variable and crossed generations. For larvae from Williamstown, parental larval size positively affected initial colony growth and larval size in the next generation. For larvae from the other population, parental larval size positively affected colony fecundity and negatively affected larval size in the next generation. Traditionally, exogenous factors have been viewed as the sole source of variation in offspring size/fitness relationship but these results show that endogenous factors (maternal source population) can also cause variation in this crucial relationship. It appears offspring size effects can be highly variable among populations and organisms can adapt to local conditions without changing the size of their offspring.