No gene flow between wing forms and clonal reproduction by males in the long-winged form of the ant <Emphasis Type="Italic">Vollenhovia emeryi</Emphasis>

In Vollenhovia emeryi, males produced by the short-winged queens (S males) have the same genotype of genomic opsin gene as the long-winged queens (L queens) rather than the cohabiting S queens. This fact suggests that either one of the following two events might have occurred, (1) a recent gene flow between the S males and the L queens or (2) a past hybridization event between them. In order to test these hypotheses, we analyzed the nuclear genome and the mitochondrial DNA of L males, L queens, and sperm from the L queens’ spermathecae. Results showed that genotype frequencies differed significantly between L males and L queens and between the sperm and L queens. Mitochondrial haplotypes of the sperm were consistent with their queen. These indicate that (1) queens of the L form mated only with the males that have been produced by L queens, and thus there is no gene flow from S males to L queens; (2) males of the L-form clonally produce sons as males of the S-form do; and (3) genotype similarity between S males and L queens indicates a past hybridization event.     

A Single Genetic Determinant that Prevents Sex Reversal in C57BL-YPOS Congenic Mice

Sex determination in the mammalian embryo begins with the activation of a gene on the Y chromosome which triggers a cascade of events that lead to male development. The mechanism by which this gene, designated SRY in humans and Sry in mice (sex determining region of the Y chromosome), is activated remains unknown. Likewise, the downstream target genes for Sry remain unidentified at present. C57BL mice carrying a Y chromosome from Mus musculus musculus or molossinus develop normally as males. In contrast, C57BL/6 mice with the Y chromosome from M. m. domesticus often show sex reversal, i.e., develop as XY females. It has been documented that C57BL mice with the Y chromosome from Poschiavinus (Y^POS), a domesticus subtype, always develop as females or hermaphrodites. This suggests that a C57BL gene either up- or downstream of Sry is ineffective in interacting with Sry, which then compromises the processes that lead to normal male sex development. Nonetheless, by selective breeding, we have been able to generate a sex reversal-resistant C57BL/6-congenic strain of mice in which the XY^POS individuals consistently develop as normal males with bilateral testes. Because the resistance to sex reversal was transferred from strain 129S1/Sv (nonalbino) by simple selection over 13 backcross generations, it is inferred that a single autosomal gene or chromosomal region confers resistance to the sex reversal that would otherwise result. XY^POS normal males generated in these crosses were compared to XY^POS abnormal individuals and to C57BL/6 controls for sexual phenotype, gonadal weight, serum testosterone, and major urinary protein (MUP) level. A clear correlation was found among phenotypic sex, MUP level, and testis weight in the males and in the incompletely masculinized XY^POS mice. The fully masculinized males of the congenic strain resemble C57BL/6 males in the tested parameters. DNA analysis confirmed that these males, in fact, carry the Y^POS Sry gene.     

Locally adapted traits maintained in the face of high gene flow

Gene flow between phenotypically divergent populations can disrupt local adaptation or, alternatively, may stimulate adaptive evolution by increasing genetic variation. We capitalised on historical Trinidadian guppy transplant experiments to test the phenotypic effects of increased gene flow caused by replicated introductions of adaptively divergent guppies, which were translocated from high‐ to low‐predation environments. We sampled two native populations prior to the onset of gene flow, six historic introduction sites, introduction sources and multiple downstream points in each basin. Extensive gene flow from introductions occurred in all streams, yet adaptive phenotypic divergence across a gradient in predation level was maintained. Descendants of guppies from a high‐predation source site showed high phenotypic similarity with native low‐predation guppies in as few as ~12 generations after gene flow, likely through a combination of adaptive evolution and phenotypic plasticity. Our results demonstrate that locally adapted phenotypes can be maintained despite extensive gene flow from divergent populations.     

Cryptic sex? Estimates of genome exchange in unisexual mole salamanders (Ambystoma sp.)

Cryptic sex has been argued to explain the exceptional longevity of certain parthenogenetic vertebrate lineages, yet direct measurements of genetic exchange between sexual and apparently parthenogenetic forms are rare. Female unisexual mole salamanders (Ambystoma sp.) are the oldest known unisexual vertebrate lineage (~5 million years), and one hypothesis for their persistence is that allopolyploid female unisexuals periodically exchange haploid genomes ‘genome exchange’ during gynogenetic reproduction with males from sympatric sexual species. We test this hypothesis by using genome‐specific microsatellite DNA markers to estimate the rates of genome exchange between sexual males and unisexual females in two ponds in NE Ohio. We also test the prediction that levels of gene flow should be higher for ‘sympatric’ (sexual males present) genomes in unisexuals compared to ‘allopatric’ (sexual males absent) unisexual genomes. We used a model testing framework in the coalescent‐based program MIGRATE‐N to compare models where unidirectional gene flow is present and absent between sexual species and unisexuals. As predicted, our results show higher levels of gene flow between sexuals and sympatric unisexual genomes compared to lower (likely artefactual) levels of gene flow between sexuals and allopatric unisexual genomes. Our results provide direct evidence that genome exchange between sexual and unisexual Ambystoma occurs and demonstrate that the magnitude depends on which sexual species are present. The relatively high levels of gene flow suggest that unisexuals must be at a selective advantage over sexual forms so as to avoid extinction due to genetic swamping through genome exchange.     

Downstairs gene flow: the effects of a linear sequence of waterfalls on the only population of the endangered minnow Astyanax xavante

The aim of this study was to estimate the genetic diversity and structure of the only known population of minnow Astyanax xavante, which inhabits a stretch of river including several waterfalls. The F_ST values among the samples were not significant, except between two populations separated by a 30 m waterfall. Nevertheless, haplotype and nucleotide diversity increased in the downstream direction, indicating that gene flow is unidirectional, which indicates this genetic pattern as downstairs gene flow, as it has the effect of increasing genetic diversity in the downstream direction.     

Tracing the emergence of a novel sex-determining gene in medaka, Oryzias luzonensis

Three sex-determining (SD) genes, SRY (mammals), Dmy (medaka), and DM-W (Xenopus laevis), have been identified to date in vertebrates. However, how and why a new sex-determining gene appears remains unknown, as do the switching mechanisms of the master sex-determining gene. Here, we used positional cloning to search for the sex-determining gene in Oryzias luzonensis and found that GsdfY (gonadal soma derived growth factor on the Y chromosome) has replaced Dmy as the master sex-determining gene in this species. We found that GsdfY showed high expression specifically in males during sex differentiation. Furthermore, the presence of a genomic fragment that included GsdfY converts XX individuals into fertile XX males. Luciferase assays demonstrated that the upstream sequence of GsdfY contributes to the male-specific high expression. Gsdf is downstream of Dmy in the sex-determining cascade of O. latipes, suggesting that emergence of the Dmy-independent Gsdf allele led to the appearance of this novel sex-determining gene in O. luzonensis.     

Male discrimination of female mucous trails permits assortative mating in a marine snail species

Recent research has shown the potential for nonallopatric speciation, but we lack an adequate understanding of the mechanisms of prezygotic barriers and how these evolve in the presence of gene flow. The marine snail Littorina saxatilis has distinct ecotypes in different shore microhabitats. Ecotypes hybridize in contact zones, but gene flow is impeded by assortative mating. Earlier studies have shown that males and females of the same ecotype copulate for longer than mates of different ecotype. Here we report a new mechanism that further contributes to reproductive isolation between ecotypes in the presence of gene flow. This mechanism is linked to the ability of males to track potential partners by following their mucous trail. We show that cliff ecotype males follow the trails of females of the same ecotype for longer than females of the alternate (boulder) ecotype. In addition, cliff males are more likely to follow the mucous trail in the correct direction if the trail is laid by a cliff-female. The capacity to discriminate the ecotype of female mucous trails combined with differential copulation times creates a strong prezygotic reproductive barrier between ecotypes of L. saxatilis that reduces gene flow from cliff to boulder ecotypes by >/=80%.     

AN UNUSUAL PATTERN OF GENE FLOW BETWEEN THE TWO SOCIAL FORMS OF THE FIRE ANT SOLENOPSIS INVICTA

Uncertainty over the role of shifts in social behavior in the process of speciation in social insects has stimulated interest in determining the extent of gene flow between conspecific populations differing in colony social organization. Allele and genotype frequencies at 12 neutral polymorphic protein markers, as well as the numbers of alleles at the sex-determining locus (loci), are shown here to be consistent with significant ongoing gene flow between two geographically adjacent populations of Solenopsis invicta that differ in colony queen number. Data from a thirteenth protein marker that is under strong differential selection in the two social forms confirm that such gene flow occurs. Data from this selected locus, combined with knowledge of the reproductive biology of the two social forms, further suggest that interform gene flow is largely unidirectional and mediated through males only. This unusual pattern of gene flow results from the influence of the unique social enviroments of the two forms on the behavior of workers and on the reproductive physiology of sexuals.     

Asymmetric isolating barriers between different microclimatic environments caused by low immigrant survival

Spatially variable selection has the potential to result in local adaptation unless counteracted by gene flow. Therefore, barriers to gene flow will help facilitate divergence between populations that differ in local selection pressures. We performed spatially and temporally replicated reciprocal field transplant experiments between inland and coastal habitats using males of the common blue damselfly (Enallagma cyathigerum) as our study organism. Males from coastal populations had lower local survival rates than resident males at inland sites, whereas we detected no differences between immigrant and resident males at coastal sites, suggesting asymmetric local adaptation in a source–sink system. There were no intrinsic differences in longevity between males from the different environments suggesting that the observed differences in male survival are environment-dependent and probably caused by local adaptation. Furthermore, the coastal environment was found to be warmer and drier than the inland environment, further suggesting local adaptation to microclimatic factors has lead to differential survival of resident and immigrant males. Our results suggest that low survival of immigrant males mediates isolation between closely located populations inhabiting different microclimatic environments.     

Disruptive sexual selection on male nuptial coloration in an experimental hybrid population of cichlid fish

Theory suggests that genetic polymorphisms in female mating preferences may cause disruptive selection on male traits, facilitating phenotypic differentiation despite gene flow, as in reinforcement or other models of speciation with gene flow. Very little experimental data have been published to test the assumptions regarding the genetics of mate choice that such theory relies on. We generated a population segregating for female mating preferences and male colour dissociated from other species differences by breeding hybrids between species of the cichlid fish genus Pundamilia. We measured male mating success as a function of male colour. First, we demonstrate that non-hybrid females of both species use male nuptial coloration for choosing mates, but with inversed preferences. Second, we show that variation in female mating preferences in an F2 hybrid population generates a quadratic fitness function for male coloration suggestive of disruptive selection: intermediate males obtained fewer matings than males at either extreme of the colour range. If the genetics of female mate choice in Pundamilia are representative for those in other species of Lake Victoria cichlid fish, it may help explain the origin and maintenance of phenotypic diversity despite some gene flow.     

Strong cryptic prezygotic isolation despite lack of behavioral isolation between sympatric host races of the leaf beetle Lochmaea capreae

One of the major goals in speciation research is to understand which isolation mechanisms form the first barriers to gene flow. This requires examining lineages that are still in the process of divergence or incipient species. Here, we investigate the presence of behavioral and several cryptic barriers between the sympatric willow and birch host races of Lochmaea capreae. Behavioral isolation did not have any profound effect on preventing gene flow. Yet despite pairs mating indiscriminately, no offspring were produced from the heterospecific matings between birch females and willow males due to the inability of males to transfer sperm to females. We found evidence for differences in genital morphology that may contribute to failed insemination attempts during copulation. The heterospecific matings between willow females and birch males resulted in viable offspring. Yet fecundity and hatchability was remarkably reduced, which is likely the result of lower efficiency in sperm transportation and storage and lower survival of sperm in the foreign reproductive tract. Our results provide evidence for the contribution of several postmating‐prezygotic barriers that predate behavioral isolation and act as primary inhibitors of gene flow in this system. This is a surprising, yet perhaps often overlooked feature of barriers acting early in sympatric speciation process.     

Crayfish populations genetically fragmented in streams impounded for 36–104 years

1. Dams and their associated impoundments may restrict dispersal and gene flow among populations of numerous freshwater species within stream networks, leading to genetic isolation. This can reduce effective population sizes and genetic diversity, increasing the risk of local extinction.
2. We studied crayfishes from multiple up- and downstream sites in three impounded and two unimpounded streams in the Bear Creek and Cahaba River drainages, Alabama, U.S.A. Using mitochondrial DNA (cytochrome oxidase subunit I gene) sequence data generated from population-level sampling of two abundant native crayfishes, Faxonius validus and Faxonius erichsonianus (Decapoda: Cambaridae), we assessed species’ spatial genetic structure and genetic diversity, estimated the magnitude and directionality of gene flow, and compared results between the species.
3. For both species, levels of genetic diversity (number of haplotypes, and haplotypic and nucleotide diversity) were the same or higher in impounded compared to unimpounded streams. Conversely, crayfish populations in up- and downstream sections of unimpounded streams displayed high genetic similarity and bidirectional gene flow, whereas in impounded streams, crayfish populations typically had greater up- and downstream genetic differentiation and predominantly unidirectional, downstream gene flow.
4. Although impoundments were associated with lower connectivity between up- and downstream sections for F. validus and F. erichsonianus, the magnitude of genetic effects was species-specific, with greater differentiation between F. validus populations up- and downstream of impoundments.
5. In an ecologically short timeframe, impoundments appear to have fragmented stream crayfish populations, and even species with relatively high abundances and large ranges had lower gene flow among populations in impounded streams compared to unimpounded streams. In addition, feedbacks between genetic and demographic effects on fragmented populations may decrease the probability of long-term persistence.

     

Looking to the past and the future: were the Madeira River rapids a geographical barrier to the boto (Cetacea: Iniidae)?

In the present study we tested if a series of 18 rapids on the upper Madeira River form an effective barrier to gene flow, and in particular if they delimit the distribution of the boto Inia boliviensis—which it is believed to occurs only in the Bolivian sub-basin, above the rapids—and I. geoffrensis, which occurs throughout the Amazon basin and below the upper Madeira River rapids. We analyzed 125 individuals from the Madeira River basin sampled from upstream and downstream of the rapids. As the two species are morphologically similar, we used diagnostic molecular characters from known reference specimens to assign individuals to species. We observed that all individuals of Inia from the Bolivian sub-basin up to almost the mouth of the Madeira River belong to the species I. boliviensis. Therefore we concluded that the rapids do not delimit the distribution of I. boliviensis upstream and I. geoffrensis downstream of the rapids as previously hypothesized. Since we registered I. boliviensis along almost the entire length of the Madeira River, we estimated gene flow, time of divergence and effective population sizes of the upstream (Bolivian) and downstream (Madeira River) groups of I. boliviensis using IMa2. We concluded that gene flow is uni-directional from the upstream to the downstream group. Divergence time between the two groups was estimated to have occurred ~122 thousand years ago. The coalescent effective population size for the upstream group was estimated at ~131 thousand individuals, while for the downstream group it was estimated at ~102 thousand individuals. Recently two dams have been constructed in the region of the rapids; neither has a mechanism that will maintain connectivity between the upstream and downstream regions, and together with anthropogenic alterations to the hydrodynamic regime and ecology of the river will likely pose serious long-term and short-term consequences for I. boliviensis and other aquatic taxa.     

Courtship behavior,swimming performance,and microhabitat use of Trinidadian guppies

Synopsis We document differences in the use of microhabitats, male courtship behavior, and swimming performance of populations from headwater and downstream sites in two rivers of the Oropuche drainage in Trinidad. Guppies from headwater sites used microhabitats with higher water velocities, had a higher swimming performance, and were less patchily distributed than guppies from downstream sites. Although males from the headwater and downstream sites had similar display rates, males from headwater sites displayed in microhabitats with higher velocities (riffles) whereas males in downstream sites courted in still pools. Subtle effects of female choice maintain the honesty of male courtship behavior in various microhabitats. In downstream sites, where predators impose a survivorship cost on ornamental males, swimming performance was positively correlated with area of carotenoid ornamentation. In headwater sites, males frequently displayed in fast-flowing water, thus paid a higher metabolic cost of courtship. Interactions between characteristics of the physical habitat and predation pressure not only affect the distribution of guppies, but also have subtle effects on the types of condition-dependent traits favored by females.     

Fine-scale differentiation between sockeye salmon ecotypes and the effect of phenotype on straying

A long-standing goal of evolutionary biology is to understand the factors that drive population divergence, local adaptation and speciation. In particular, the effect of selection against dispersers on gene flow and local adaptation has attracted interest, although empirical data on phenotypic characters of dispersers are scarce. Here, we used genetic and phenotypic data from beach and creek ecotypes of sockeye salmon (Oncorhynchus nerka) in Little Togiak Lake, Alaska, to examine the relationship between gene flow and phenotypic and genetic differentiation. Despite close geographic proximity, both genetic and phenotypic differentiation between beach and creek fish was high and significant in all sampling years, with beach males having deeper bodies than creek males. Strays, or fish that did not return to their natal sites to spawn as determined by genetic assignment, tended to morphologically resemble the fish in the population that they joined. Male strays from beaches to creeks were shallower bodied than other beach fish, and male strays from creeks to beaches were deeper bodied than other creek males. Our results indicated that selection against strays may be moderated by the strays' phenotypic similarity to individuals in the recipient populations, but comparison of assignment results with long-term estimates of gene flow from F(ST) still suggested that strays had low reproductive success.     

Fine‐scale movements and interactions of platypuses,and the impact of an environmental flushing flow

1. The platypus is a cryptic mammal that inhabits freshwater streams and rivers of eastern Australia. Tracking the movements of wild platypuses has been notoriously difficult due to the animals' morphology and methodological limitations. Knowledge of fine‐scale movements and interactions among individuals remain particularly poorly understood, as do responses to changes in hydrology.
2. We tracked movements of 15 platypuses (six females, nine males) downstream of the Jindabyne Dam on the Snowy River, using externally attached acoustic transmitters (September–November 2017), to assess spatio‐temporal activity patterns among individuals and changes in movement and activity before and after an environmental flushing flow. As the study took place during the breeding season, we expected to observe overlap in area of activity among males and females, but not among males due to increased territoriality during these months. We also anticipated that a large flow event would impact their activity and foraging behaviour, possibly displacing platypuses downstream.
3. Overlaps in area of activity and temporal co‐occurrence within a pool varied among individuals, with two resident males exhibiting some spatial overlap of activity and varying temporal co‐occurrence, despite tracking during the breeding season. All six tracked females were captured in the same pool and appeared to be residents, possibly highlighting preferences for certain habitats during the breeding months.
4. We found no evidence that the movements of adult platypuses were affected by an environmental flushing flow, with no significant changes to area of activity, number of detections, or daily range of movements. However, foraging duration increased in the week after the flow, possibly associated with increased prey availability.
5. These findings suggest that territoriality between males during and after the breeding season may depend on platypus density and resource availability and that pools with high resource availability may support several breeding females.

     

Dorsal skin color patterns among southern right whales (Eubalaena australis): genetic basis and evolutionary significance.

Distribution and inheritance of dorsal skin color markings among two populations of southern right whales (Eubalaena australis) suggest that two genes influence dorsal skin color. The grey-morph and partial-grey-morph phenotypes (previously known as partial albino and grey-blaze, respectively) appear to be controlled by an X-linked gene, whereas the white blaze appears controlled by an autosomal gene (recessive phenotype). Calving intervals, calf size, and length of sighting history data suggest that partial-grey-morph, white-blaze, and black cows experience similar levels of reproductive success. Grey-morph cows (XgXg) are rare or absent in the two populations, but this was not unexpected given observed population frequencies of grey-morph males (XgY) and partial-grey-morph females (XGXg). The proportion of partial-grey-morph calves produced by black cows (XGXG) suggests that the reproductive success of grey-morph males was equal to that of black males, however, larger sample sizes are required to determine whether grey-morph males tend to have shorter sighting histories. The reproductive success of white-blaze males appeared similar to that of black males among whales off Argentina. There were significantly fewer white-blaze calves than expected off South Africa, which could be due to white-blaze males experiencing reduced reproductive success or to sighting blases that result in white-marked calves being misidentified as black calves. The relative frequencies of both types of dorsal color markings varied between the South African and Argentinian right whale populations, suggesting limited nuclear gene flow between these populations; analyses using other nuclear markers are under way to confirm the extent of gene flow.     

Is Niagara Falls a barrier to gene flow in riverine fishes? A test using genome‐wide SNP data from seven native species

Since the early Holocene, fish population genetics in the Laurentian Great Lakes have been shaped by the dual influences of habitat structure and post‐glacial dispersal. Riverscape genetics theory predicts that longitudinal habitat corridors and unidirectional downstream water‐flow drive the downstream accumulation of genetic diversity, whereas post‐glacial dispersal theory predicts that fish genetic diversity should decrease with increasing distance from glacial refugia. This study examines populations of seven native fish species codistributed above and below the 58 m high Niagara Falls – a hypothesized barrier to gene flow in aquatic species. A better understanding of Niagara Falls’ role as a barrier to gene flow and dispersal is needed to identify drivers of Great Lakes genetic diversity and guide strategies to limit exotic species invasions. We used genome‐wide SNPs and coalescent models to test whether populations are: (a) genetically distinct, consistent with the Niagara Falls barrier hypothesis; (b) more genetically diverse upstream, consistent with post‐glacial expansion theory, or downstream, consistent with the riverscape habitat theory; and (c) have migrated either upstream or downstream past Niagara Falls. We found that genetic diversity is consistently greater below Niagara Falls and the falls are an effective barrier to migration, but two species have probably dispersed upstream past the falls after glacial retreat yet before opening of the Welland Canal. Models restricting migration to after opening of the Welland Canal were generally rejected. These results help explain how river habitat features affect aquatic species’ genetic diversity and highlight the need to better understand post‐glacial dispersal pathways.