Moreover, old gene-poor Y chromosomes that are tens of millions of years old, such as the Drosophila Y actually show a net rate of gene gain rather than gene loss . In contrast to the lability of sex determination mechanisms in some groups, eutherian mammals, birds and many insects exhibit virtually no variation in how sex is determined Figure 3.
A sex-determination system is a x0 sex determination systems in Baton Rouge system that determines the development of sexual characteristics in an organism. Meiotic drive also called segregation distortion : a system in which genetic elements termed segregation distorters bias the proportion of gametes that carry them, resulting in over- or under-representation of one gametic type i.
Turnover of sex chromosomes In species with genotypic sex determination, the chromosome pair that determines sex can change rapidly x0 sex determination systems in Baton Rouge time. Table 1. Charlesworth B The evolution of chromosomal sex determination and dosage compensation.
In many species, the line between GSD and ESD is blurred, with certain environments altering the otherwise genetically determined sex of developing offspring . Separate sexes may evolve from hermaphrodites either by gradual increases in sex-specific investment or rapidly by the appearance of male- or female-sterility mutations Figure 2.
Compared to asexual reproduction, in which cells can simply create carbon copies of themselves, sexual reproduction allows for the introduction of genetic diversity into a population. Saumitou-Laprade P, Cuguen J, Vernet P Cytoplasmic male sterility in plants: molecular evidence and the nucleocytoplasmic conflict.
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In the chance that is not your source material: citations would be appreciated. However, theory predicts that the rate of gene decay on the Y decreases over evolutionary time and should halt on an old, gene-poor Y chromosome  , . Sunderland, Mass. They are then thought to differentiate through an inevitable and irreversible process in which recombination between X and Y chromosomes is shut down and the Y degenerates see Figure 1.
Mating types might have evolved to orchestrate the developmental transition from the haploid to the diploid phase of the life cycle  ,  : plus and minus gametes express complementary transcription factors, encoded by different alleles at the MAT locus; these combine in the zygote into heterodimers that silence the genes expressed in the haploid phase and switch on the diploid program.
Separate sexes may evolve from hermaphrodites either by gradual increases in sex-specific investment or rapidly by the appearance of male- or female-sterility mutations Figure 2.