Kohlmann, K. | bioRxiv Science

bioRxiv2020

A 180 My-old female-specific genome region in sturgeon reveals the oldest known vertebrate sex determining system with undifferentiated sex chromosomes

Several hypotheses explain the prevalence of undifferentiated sex chromosomes in poikilothermic vertebrates. Turnovers change the master sex determination gene, the sex chromosome or the sex determination system (e.g. XY to WZ). Jumping master genes stay main triggers but translocate to other chromosomes. Occasional recombination (e.g. in sex-reversed females) prevents sex chromosome degeneration. Recent research has uncovered conserved heteromorphic or even homomorphic sex chromosomes in several clades of non-avian and non-mammalian vertebrates. Sex determination in sturgeons (Acipenseridae) has been a long-standing basic biological question, linked to economical demands by the caviar-producing aquaculture. Here, we report the discovery of a sex-specific sequence from sterlet (Acipenser ruthenus). Using chromosome-scale assemblies and pool-sequencing, we first identified a ~16 kb female-specific region. We developed a PCR-genotyping test, yielding female-specific products in six species, spanning the entire phylogeny with the most divergent extant lineages (A. sturio, A. oxyrinchus vs. A. ruthenus, Huso huso), stemming from an ancient tetraploidization. Similar results were obtained in two octoploid species (A. gueldenstaedtii, A. baerii). Conservation of a female-specific sequence for a long period, representing 180 My of sturgeon evolution, and across at least one polyploidization event, raises many interesting biological questions. We discuss a conserved undifferentiated sex chromosome system with a ZZ/ZW-mode of sex determination and potential alternatives.

evolutionary biology↗

bioRxiv2019

Intraspecific population admixture of a top piscivore correlates with anthropogenic alteration of freshwater ecosystems

Conservation of local genetic diversity is an important policy objective, but intraspecific genetic diversity can be transformed by natural ecological processes associated with anthropogenic changes in ecosystems. Environmental changes and a strong interconnection of drainage systems impact freshwater biodiversity from gene to population level. Populations can either become extinct or expand their range and accompanying secondary contacts can lead to genetic admixture. We investigated how the genetic population structure and the patterns of genetic admixture of Esox lucius L. (the northern pike) vary with the type of ecosystem and the integrity of the ecosystem assessed by measures under the European Water Framework Directive. The pike inhabits river, lake and brackish water ecosystems, where it is confronted with different ecological disturbances. We analysed 1,384 pike samples from the North, Baltic and Black Sea drainages and differentiated between metapopulations from each hydrogeographic region using genotypes from 15 microsatellites and mitochondrial cyt b sequences. Individual populations showed signs of genetic admixture ranging from almost zero to complete replacement by foreign genotypes. Hierarchical general linear modeling revealed a highly significant positive association of the degree of genetic admixture with decreasing ecological status. This may mean that populations in disturbed environments are more prone to influences by foreign genotypes or, alternatively, increased genetic admixture may indicate adaptation to rapid environmental changes. Regardless of the underlying mechanisms, our results suggest that anthropogenic alterations of natural freshwater ecosystems can influence genetic structures, which may lead to a large-scale reduction of intraspecific genetic diversity.

evolutionary biology↗