Survival of Polyploid hybrid salamander embryos

Background

Animals with polyploid, hybrid nuclei offer a challenge for models of gene expression and regulation during embryogenesis. To understand how such organisms proceed through development, we examined the timing and prevalence of mortality among embryos of unisexual salamanders in the genus Ambystoma.

Results

Our regional field surveys suggested that heightened rates of embryo mortality among unisexual salamanders begin in the earliest stages of embryogenesis. Although we expected elevated mortality after zygotic genome activation in the blastula stage, this is not what we found among embryos which we reared in the laboratory. Once embryos entered the first cleavage stage, we found no difference in mortality rates between unisexual salamanders and their bisexual hosts. Our results are consistent with previous studies showing high rates of unisexual mortality, but counter to reports that heightened embryo mortality continues throughout embryo development.

Conclusions

Possible causes of embryonic mortality in early embryogenesis suggested by our results include abnormal maternal loading of RNA during meiosis and barriers to insemination. The surprising survival rates of embryos post-cleavage invites further study of how genes are regulated during development in such polyploid hybrid organisms.

A family study to examine clonal diversity in unisexual salamanders (genus Ambystoma)

Unisexual Ambystoma are the oldest known unisexual vertebrates and comprise a lineage of eastern North American all female salamanders that reproduce by stealing sperm from as many as five normally bisexual congeneric species. The sperm may be used to only stimulate egg development by gynogenesis but can be incorporated in the zygote to elevate the ploidy level or to replace one of the female’s haploid genomes. This flexible and unique reproductive system, termed kleptogenesis, is investigated using a microsatellite examination of 988 offspring from 14 unisexual mothers. All mothers produced clonal and ploidy-elevated offspring. Genome replacement and multiple paternity are confirmed for the first time in unisexual Ambystoma. Microsatellite mutations were found in all five microsatellite loci and the estimated microsatellite mutation rate varied by locus and by genome. Clonal variation is attributed to the inclusion of sperm donors’ haploid genomes for ploidy elevation, genome replacement, mutations, and natural selection.

Effect of a locally adapted genome on environmentally induced epigenetic variation

Both genetic variation and environmentally induced epigenetic changes allow organisms to persist through the heterogeneity of their habitats. Selection on genetic variation can promote local adaptation of populations. However, in absence of genetic variation, clonal organisms mostly rely on epigenetics to respond to environmental heterogeneity. We used the potential of unisexual organisms in incorporating their host genome, to empirically assess whether the presence of a locally adapted genome affects environmentally induced epigenetic changes in clonal organisms. We addressed this problematic by using unisexual lineages of the kleptogen vertebrate Ambystoma laterale-jeffersonianum complex that can optionally incorporate genetic material from locally adapted sexual hosts through genomic exchanges. More specifically, we compared environmentally induced epigenetic changes between lineages strictly reproducing clonally vs. those incorporating a locally adapted genome. The results revealed that both lineage and sample site components, as well as their interaction, affected epigenetic variation. When lineages were analysed separately, differences among sample sites were only detected in lineages impervious to genomic exchanges. Sample sites had no significant effect on the epigenetic variation of lineages that performed genomic exchanges. These results suggest that environmentally induced epigenetic variation among sites depends more on the lack of locally adapted alleles than on the level of genetic variation.

Influence of genome and bio-ecology on the prevalence of genome exchange in unisexuals of the Ambystoma complex

Background

Unisexuals of the blue-spotted salamander complex are thought to reproduce by kleptogenesis. Genome exchanges associated with this sperm-dependent mode of reproduction are expected to result in a higher genetic variation and multiple ploidy levels compared to clonality. However, the existence of some populations exclusively formed of genetically identical individuals suggests that factors could prevent genome exchanges. This study aimed at assessing the prevalence of genome exchange among unisexuals of the Ambystoma laterale-jeffersonianum complex from 10 sites in the northern part of their distribution.

Results

A total of 235 individuals, including 207 unisexuals, were genotyped using microsatellite loci and AFLP. Unisexual individuals could be sorted in five genetically distinct groups, likely derived from the same paternal A. jeffersonianum haplome. One of these groups exclusively reproduced clonally, even when found in sympatry with lineages presenting signature of genome exchange. Genome exchange was site-dependent for another group. Genome exchange was detected at all sites for the three remaining groups.

Conclusion

Prevalence of genome exchange appears to be associated with ecological conditions such as availability of effective sperm donors. Intrinsic genomic factors may also affect this process, since different lineages in sympatry present highly variable rate of genome exchange. The coexistence of clonal and genetically diversified lineages opens the door to further research on alternatives to genetic variation.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1200-7) contains supplementary material, which is available to authorized users.

Genetic and genomic interactions of animals with different ploidy levels

Polyploid animals have independently evolved from diploids in diverse taxa across the tree of life. We review a few polyploid animal species or biotypes where recently developed molecular and cytogenetic methods have significantly improved our understanding of their genetics, reproduction and evolution. Mitochondrial sequences that target the maternal ancestor of a polyploid show that polyploids may have single (e.g. unisexual salamanders in the genus Ambystoma) or multiple (e.g. parthenogenetic polyploid lizards in the genus Aspidoscelis) origins. Microsatellites are nuclear markers that can be used to analyze genetic recombinations, reproductive modes (e.g. Ambystoma) and recombination events (e.g. polyploid frogs such as Pelophylax esculentus). Hom(e)ologous chromosomes and rare intergenomic exchanges in allopolyploids have been distinguished by applying genome-specific fluorescent probes to chromosome spreads. Polyploids arise, and are maintained, through perturbations of the 'normal' meiotic program that would include pre-meiotic chromosome replication and genomic integrity of homologs. When possible, asexual, unisexual and bisexual polyploid species or biotypes interact with diploid relatives, and genes are passed from diploid to polyploid gene pools, which increase genetic diversity and ultimately evolutionary flexibility in the polyploid. When diploid relatives do not exist, polyploids can interact with another polyploid (e.g. species of African Clawed Frogs in the genus Xenopus). Some polyploid fish (e.g. salmonids) and frogs (Xenopus) represent independent lineages whose ancestors experienced whole genome duplication events. Some tetraploid frogs (P. esculentus) and fish (Squaliusalburnoides) may be in the process of becoming independent species, but diploid and triploid forms of these 'species' continue to genetically interact with the comparatively few tetraploid populations. Genetic and genomic interaction between polyploids and diploids is a complex and dynamic process that likely plays a crucial role for the evolution and persistence of polyploid animals. See also other articles in this themed issue.

Relating hybrid advantage and genome replacement in unisexual salamanders

Unisexual vertebrates are model systems for understanding the evolution of sex. Many predominantly clonal lineages allow occasional genetic recombination, which may be sufficient to avoid the accumulation of deleterious mutations and parasites. Introgression of paternal DNA into an all-female lineage represents a one-way flow of genetic material. Over many generations, this could result in complete replacement of the unisexual genomes by those of the donor species. The process of genome replacement may be counteracted by contemporary dispersal or by positive selection on hybrid nuclear genomes in ecotones. I present a conceptual model that relates nuclear genome replacement, positive selection on hybrids and biogeography in unisexual systems. I execute an individual-based simulation of the fate of hybrid genotypes in contact with a single host species. I parameterize these models for unisexual salamanders in the Ambystoma genus, for which the frequency of genome replacement has been a source of ongoing debate. I find that, if genome replacement occurs at a rate greater than 1/10,000 in Ambystoma, then there must be compensating positive selection in order to maintain observed levels of hybrid nuclei. Future researchers studying unisexual systems may use this framework as a guide to evaluating the hybrid superiority hypothesis.