This variation could be in part driven by the generally higher rates of mutation in males Wilson Sayres and Makova that would cause Y chromosomes to accumulate mutations and degenerate faster than W chromosomes.
Itoh Yet al. Evidence of sex-bias in gene expression in the brain transcriptome of two populations of rainbow trout Oncorhynchus mykiss with divergent life histories. Mechanisms of Recombination Suppression. Otto SPet al. This is because their retention within the genome is required in order to maintain dosage balance.
Google Scholar Crossref.
Female gametes, or eggs, however, contain only the X sex chromosome and are homogametic. If, then, the male element prevails it draws the female element into itself, but if it is prevailed over it changes into the opposite or is destroyed. The hermaphrodite sex is estimated to have arisen only years ago, post-domestication of the plant.
Sex chromosomes are most common in bryophytesrelatively common in vascular plants and unknown in ferns and lycophytes. The high prevalence of autopolyploidy in plants also impacts the structure of their sex chromosomes.
Cytogenetics of the endemic New Zealand frog, Leiopelma hochstetteri : extraordinary supernumerary chromosome variation and a unique sex-chromosome system. When present, the development of separate sexes requires a tightly regulated genetic cascade, as future reproductive potential relies heavily on the presence of primary sexual characteristics.
Gamble Tet al. Mol Biol Evol.
Alternatively, the buildup of mutations on the Y chromosome may be faster than what rare recombination events between the X and Y can counter. Dosage compensation was originally thought to occur across the entirety of the X or Z chromosome, evolving primarily in response to selection for hyperexpression in the heterogametic sex in order to achieve parity between the sex chromosomes and the autosomes Ohno In addition, like ohnologs, drift could play a role in establishing dosage compensation-like patterns, and take an extended period of time to become established Gout and Lynch , as the sex-shared copy drifts to higher expression levels while the sex-specific copy progressively degrades.
Google Preview. Open in new tab Download slide.