The Diversity and Evolution of Sex Chromosomes in Frogs

Frogs are ideal organisms for studying sex chromosome evolution because of their diversity in sex chromosome differentiation and sex-determination systems. We review 222 anuran frogs, spanning ~220 Myr of divergence, with characterized sex chromosomes, and discuss their evolution, phylogenetic distribution and transitions between homomorphic and heteromorphic states, as well as between sex-determination systems. Most (~75%) anurans have homomorphic sex chromosomes, with XY systems being three times more common than ZW systems. Most remaining anurans (~25%) have heteromorphic sex chromosomes, with XY and ZW systems almost equally represented. There are Y-autosome fusions in 11 species, and no W-/Z-/X-autosome fusions are known. The phylogeny represents at least 19 transitions between sex-determination systems and at least 16 cases of independent evolution of heteromorphic sex chromosomes from homomorphy, the likely ancestral state. Five lineages mostly have heteromorphic sex chromosomes, which might have evolved due to demographic and sexual selection attributes of those lineages. Males do not recombine over most of their genome, regardless of which is the heterogametic sex. Nevertheless, telomere-restricted recombination between ZW chromosomes has evolved at least once. More comparative genomic studies are needed to understand the evolutionary trajectories of sex chromosomes among frog lineages, especially in the ZW systems

Experimental sexual selection reveals rapid evolutionary divergence in sex-specific transcriptomes and their interactions following mating

Work was supported by the Natural Environment Research Council (NE/I014632/1 to M.G.R., A.R.C., and R.R.S), the Natural Environment Research Council Biomolecular Analysis Facility (NBAF654 to M.G.R), and the Swedish Research Council (Vetenskapsrådet; 2018-04598 to R.R.S).Postcopulatory interactions between the sexes in internally fertilizing species elicits both sexual conflict and sexual selection. Macroevolutionary and comparative studies have linked these processes to rapid transcriptomic evolution in sex-specific tissues and substantial transcriptomic postmating responses in females, patterns of which are altered when mating between reproductively isolated species. Here we test multiple predictions arising from sexual selection and conflict theory about the evolution of sex-specific and tissue-specific gene expression and the postmating response at the microevolutionary level. Following over 150 generations of experimental evolution under either reduced (enforced monogamy) or elevated (polyandry) sexual selection in Drosophila pseudoobscura, we found a substantial effect of sexual selection treatment on transcriptomic divergence in virgin male and female reproductive tissues (testes, male accessory glands, the female reproductive tract and ovaries). Sexual selection treatment also had a dominant effect on the postmating response, particularly in the female reproductive tract ? the main arena for sexual conflict - compared to ovaries. This effect was asymmetric with monandry females typically showing more postmating responses than polyandry females, with enriched gene functions varying across treatments. The evolutionary history of the male partner had a larger effect on the postmating response of monandry females, but females from both sexual selection treatments showed unique patterns of gene expression and gene function when mating with males from the alternate treatment. Our microevolutionary results mostly confirm comparative macroevolutionary predictions on the role of sexual selection on transcriptomic divergence and altered gene regulation arising from divergent coevolutionary trajectories between sexual selection treatments.Publisher PDFPeer reviewe

3D-Modeling for the Analysis of Range Data

In this paper a new method of modeling 30-objects for the interpretation of range images is described.In range data it is possible to detect surface patches, edges and 30- vertices by local and therefore fast operations. Recognition of an object is done by associating these structures with one element of a set of possible objects. For that it is necessary to use an adequate description of objects which supports fast matching. In this paper a model is presented, which represents an object by surfaces, edges, vertices and neighborhood relationships. The representation of the objects is restricted on those surfaces, which are easy to detect by the used range sensor.A further requirement is a fast and easy interactive construction of object models which assists human demands and thus reduces the costs of providing and maintenance of the object database

The Inexorable Spread of a Newly Arisen Neo-Y Chromosome

A newly arisen Y-chromosome can become established in one part of a species range by genetic drift or through the effects of selection on sexually antagonistic alleles. However, it is difficult to explain why it should then spread throughout the species range after this initial episode. As it spreads into new populations, it will actually enter females. It would then be expected to perform poorly since it will have been shaped by the selective regime of the male-only environment from which it came. We address this problem using computer models of hybrid zone dynamics where a neo-XY chromosomal race meets the ancestral karyotype. Our models consider that the neo-Y was established by the fusion of an autosome with the ancestral X-chromosome (thereby creating the Y and the ‘fused X’). Our principal finding is that sexually antagonistic effects of the Y induce indirect selection in favour of the fused X-chromosomes, causing their spread. The Y-chromosome can then spread, protected behind the advancing shield of the fused X distribution. This mode of spread provides a robust explanation of how newly arisen Y-chromosomes can spread. A Y-chromosome would be expected to accumulate mutations that would cause it to be selected against when it is a rare newly arrived migrant. The Y can spread, nevertheless, because of the indirect selection induced by gene flow (which can only be observed in models comprising multiple populations). These results suggest a fundamental re-evaluation of sex-chromosome hybrid zones. The well-understood evolutionary events that initiate the Y-chromosome's degeneration will actually fuel its range expansion

The involvement of a sex chromosome fusion and rDNA diversity in the grasshopper podisma pedestris hybrid zone

A fusion involving the X chromosome has generated a Neo-XY race from the standard acridid XOIXX karyotype in the grasshopper Podisma pedestris. The two races meet and form a hybrid zone. However the specific targets of selection responsible for its maintenance have never been found. This thesis examines the possibility of involvement of ribosomal DNA and fitness regimes driven by early sex chromosome evolution in the maintenance of the hybrid zone. rDNA was studied with fluorescent in situ hybridisation (FISH) and at the sequence level. FISH reveals an astonishing polymorphism in the distribution of rDNA loci, both within individual populations and between spatially isolated populations, allowing their subdivision into previously undetected genetic races. It is suggested that rDNA differentiation between populations could maintain the hybrid zone either by random mixing of rDNA loci, expected to generate individuals with insufficient rDNA, or by rDNA sequence incompatibility. Data from diverse experimental approaches addressed the evolution of rDNA itself. In particular, it was possible to show that rDNA loci contain multiple pseudogenes and that concerted evolution is not successful in homogenising all loci. It was also possible to demonstrate mobility of active loci and to assess the effect of rDNA copy number on the previously described genome size difference between the two races. Sex chromosome incompatibilities maintaining the hybrid zone were assessed by fitting computer simulation data to ecological data obtained from the field. The fitness reduction of the chromosomal heterozygotes for the X, or the evolution of male beneficial and female deleterious alleles on the Neo- Y, could be involved with the maintenance of the P. pedestris hybrid zone. More generally it is shown that the fusion may mark the beginning of the inevitable establishment of the Neo-XY system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The X chromosome is necessary for ovule production in Silene latifolia.

Sex chromosomes stop recombining and accumulate differences over time. In particular, genes on the chromosome restricted to the heterogametic sex degenerate and become non-functional. Here, we investigated whether or not the degeneration of a plant Y chromosome was sufficient to cause ovules containing a Y to fail to develop, thereby eliminating the possibility of YY individuals. We used two genotypic assays to determine the genotype-XX, XY, or YY-of offspring from a single fruit of an otherwise normal male XY plant of Silene latifolia. The fruit contained fewer ovules than normal pistillate flowers, produced an equal offspring sex ratio, and generated no YY offspring. The results indicate that ovaries must contain an X chromosome to develop properly. While haploid selection has slowed down Y-chromosome degeneration in S. latifolia, we find that it has progressed sufficiently to prevent the proper development of ovules, and hence prevent the presence of YY individuals