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Fig. 1 | Genome Biology

Fig. 1

From: Fruitful analysis of sex chromosomes reveals X-treme genetic diversity

Fig. 1

Sex chromosomes across taxa reveal variation in sex-linked morphology. A snapshot of the current understanding of morphological variation in sex chromosomes across diverse taxa shows how incorporating studies of young sex chromosomes provides a new perspective. The colored regions represent regions that do not recombine between the two sex chromosomes, whereas the grey color represents regions that do recombine, also known as pseudoautosomal regions (PARs). Although not exact, the approximate relative sizes of the sex chromosomes are represented. Human sex chromosomes have two small PARs at each tip. The fruit fly Drosophila melanogaster does not have recombination in males, thus there are no PARs on its sex chromosomes. Initial studies of birds in the Neoaves and Galliforms clades suggested that their sex chromosomes, despite being heteromorphic in females (ZW), mimicked the evolution of human and Drosophila male heteromorphic (XY) sex chromosomes, exhibiting highly degraded W chromosomes. However, further work in Paleognath birds, such as the emu, revealed that not all old sex chromosome systems will have a degenerated heteromorphic sex chromosome (W or Y). In contrast to birds, mammals and flies, the plants studied to date have much younger sex chromosomes, which facilitate the study of how quickly recombination suppression evolves between the sex chromosomes. The 10–20 million year old X and Y chromosomes of Silene latifolia have already experienced three recombination-suppression events, but there are small regions on the distal arm of these sex chromosomes that can still recombine. The evolutionary rate at which quick recombination suppression occurs may, however, be highly variable. The 7-million-year-old papaya sex chromosomes are largely able to recombine, with relatively small sex-specific regions. Curiously, in both papaya and S. latifolia, the Y-specific regions are larger than the X-specific regions. It is only by studying diverse taxa that we can develop truly general expectations for sex chromosome evolution

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