Models Of Sex Chromosome Evolution Research Articles

Mathematical glimpse on the Y chromosome degeneration

The Y chromosomes are genetically degenerate and do not recombine with their matching partners X. Non-recombination of XY pairs has been pointed out as the key factor for the degeneration of the Y chromosome. The aim here is to show that there is a mathematical asymmetry in sex chromosomes which leads to the degeneration of Y chromosomes even in the absence of XX and XY recombination. A model for sex-chromosome evolution in a stationary regime is proposed. The consequences of their asymmetry are analyzed and lead us to a couple of conclusions. First, Y chromosome degeneration shows up \(\sqrt{2}\) more often than X chromosome degeneration. Second, if nature prohibits female mortalities from beeing exactly \(50\%\), then Y chromosome degeneration is inevitable.

Sequence differentiation associated with an inversion on the neo-X chromosome of Drosophila americana.

Sex chromosomes originate from pairs of autosomes that acquire controlling genes in the sex-determining cascade. Universal mechanisms apparently influence the evolution of sex chromosomes, because this chromosomal pair is characteristically heteromorphic in a broad range of organisms. To examine the pattern of initial differentiation between sex chromosomes, sequence analyses were performed on a pair of newly formed sex chromosomes in Drosophila americana. This species has neo-sex chromosomes as a result of a centromeric fusion between the X chromosome and an autosome. Sequences were analyzed from the Alcohol dehydrogenase (Adh), big brain (bib), and timeless (tim) gene regions, which represent separate positions along this pair of neo-sex chromosomes. In the northwestern range of the species, the bib and Adh regions exhibit significant sequence differentiation for neo-X chromosomes relative to neo-Y chromosomes from the same geographic region and other chromosomal populations of D. americana. Furthermore, a nucleotide site defining a common haplotype in bib is shown to be associated with a paracentric inversion [In(4)ab] on the neo-X chromosome, and this inversion suppresses recombination between neo-X and neo-Y chromosomes. These observations are consistent with the inversion acting as a recombination modifier that suppresses exchange between these neo-sex chromosomes, as predicted by models of sex chromosome evolution.

Enigma of Y chromosome degeneration: neo-Y and neo-X chromosomes of Drosophila miranda a model for sex chromosome evolution.

Y chromosome degeneration is characterized by structural changes in the chromosome architecture and expansion of genetic inertness along the Y chromosome. It is generally assumed that the heteromorphic sex chromosome pair has developed from a pair of homologues. Several models have been suggested. We use the unique situation of the secondary sex chromosome pair, neo-Y and neo-X (X2), in Drosophila miranda to analyze molecular mechanisms involved in the evolutionary processes of Y chromosome degeneration. Due to the fusion of one of the autosomes to the Y chromosome (about 2 Mya), a neo-Y chromosome and a neo-X chromosome, designated X2, were formed. Thus, formerly autosomal genes are inherited now on a pair of sex chromosomes in D. miranda. Analyzing DNA sequences from the X2 and neo-Y region, we observed a massive accumulation of DNA insertions on the neo-Y chromosome. From the analysis of several insertion elements, we present compelling evidence that the first step in Y chromosome degeneration is driven by the accumulation of transposable elements, especially retrotransposons. An enrichment of these elements along an evolving Y chromosome could account for the switch from a euchromatic into a heterochromatic chromatin structure.

Further data on sex chromosomes of Lacertidae and a hypothesis on their evolutionary trend

Abstract Sex chromosomes were studied in eight species of lacertid lizards using C-banding, G-banding and restriction enzyme treatment. All of the species showed female heterogamety. The W chromosome was a microchromosome in Lacerta graeca and Ophisops elegans. Two types of W were found in Lacerta vivipara; in specimens from The Netherlands it was metacentric, whereas in specimens from Russia it was acrocentric or subtelocentric. The W chromosome was homomorphic or nearly homomorphic but completely C-banded and heterochromatic in Lacerta agilis, Podarcis hispanica, Algyroides moreoticus and A. nigropunctatus. In was only possible to find sex chromosomes using the G-banding method in Podarcis sicula. The results obtained, together with data in the literature, suggest that sex chromosomes are likely to be present in all Lacertidae and that their differentiation took place repeatedly and independently in different taxa within the family. A model for sex chromosome evolution in the family, in which the starting point was the heterochromatization of the W chromosome, is proposed.

Monotreme Genetics and Cytology and a Model for Sex-Chromosome Evolution

The protherian mammals consist of three species: the platypus, the Australian echidna and the Niugini echidna. These mammals diverged from the therian line of descent about 150-200 million years ago; hence comparisons of gene arrangements and gene control mechanisms between prototherian and therian mammals may yield significant data about gene rearrangements during mammalian evolution and about the evolution of complex genetic control systems. The chromosome complements of the three monotreme species are highly conserved. In particular, the X (or X1) chromosomes are G-band identical and share considerable G-band homology with the Y chromosomes. Replication asynchrony between X chromosomes suggests that X chromosome inactivation operates in females, and is apparently tissue- specific (as it is in marsupials), and confined to the differential region of the X (X1) chromosome (as it is in eutherian mammals). These results suggest that sex chromosome differentiation in the monotremes represents an intermediate stage in the evolution of the dimorphic sex chromosomes of therian mammals and that X-chromosome inactivation may also represent a comparatively primitive stage. Studies of gene location in the platypus using platypus-rodent cell hybrids suggested that HPRT and PGK are syntenic in the platypus, but it was not possible to assign the syntenic group to a particular chromosome. In situ hybridisation was used to assign three genes, located on the X in eutherians and marsupials, to the monotreme X. However, human X short-arm markers were found by in situ hybridisation to be autosomal in monotremes (as they are in marsupials). A model for the evolution of mammalian sex chromosome differentiation and X-chromosome inactivation is presented in which a gradual reduction of the Y chromosome, and recruitment of newly unpaired loci on the X into a system of X-chromosome inactivation, has accompanied eutherian evolution.