Box 2.
W and Y chromosome age and degeneration status
| Taxa | Sex-determination system | Degeneration of W/Y (depictively) | W/Y chromosome agea | References |
|---|---|---|---|---|
| Filamentous fungi Neurospora tetrasperma | No sex chromosomes Non-recombining mating-type chromosome. Pseudohomothallism |
75 % of mating-type chromosomes do not recombine Observed similar process of degeneration as in W/Y chromosome (including accumulation of transposable elements and gene pseudogenisation) |
3.5–5.8 MYb,c,d | Thomson and Lin (2009) |
| Plants White campion Silene latifolia |
Heteromorphic homologic XY, GSD | 20 % of genes are lost on the Y chromosome. Accumulation of transposable elements | Oldest stratum 10 MYd | Bergero and Charlesworth (2011) and Chibalina and Filatov (2011) |
| Lepidoptera Bombyx mori |
Heteromorphic WZ, GSD | No protein-coding genes on W chromosome >579 genes on previously homologous Z chromosome |
90–100 MYb,c | Fujii et al. (2010), Hara et al. (2012), Sahara et al. (2012) |
| Diptera Phorid fly Megaselia scalaris |
Homomorphic XY, GSD | Very early molecular signs of chromosome differentiation Rapid Y chromosome turnover within the species |
Variable | Traut (2010) |
| Diptera Housefly Musca domestica |
Homo- or heteromorphic XY, GSD | Early molecular signs of chromosome differentiation Y chromosome turnover within the species |
Variable | Blaser et al. (2013) |
| Diptera Drosophila albomicans |
Heteromorphic non-homologic XY, GSD | Neo-Y chromosome with no obvious signs of degeneration. ~4800 genes are still functional | 0.12 MYd | Bachtrog (2006) and Zhou et al. (2012) |
| Diptera Drosophila miranda |
Heteromorphic non-homologic XY, GSD | Neo-Y chromosome ~50 % of transposable elements, 209 putative genes left (~10 % of initial gene number) Transposable elements on 1 % of the neo-X chromosome |
1.2 MYd | Bachtrog et al. (2008) and Steinemann and Steinemann (2005) |
| Diptera Drosophila pseudoobscura |
Heteromorphic non-homologic XY, GSD | No protein-coding genes on Y chromosome (initially ~3000) | 15 MYd | Carvalho and Clark (2005) |
| Fish Family Adrianichthyidae Medaka fish Oryzias latipes |
Homomorphic homologic XY, GSD and TSD | Y chromosome degenerated only in 258 kb long sequence. The rest of the chromosome is homologous to X Mechanism of recombination of the male-specific region is preventing the spread of the non-recombining region over the Y chromosome |
10 MYd | Matsuda (2005), Kondo et al. (2006), Herpin and Schartl (2009) |
| Fish Family Gasterosteidae Stickleback fish, Gasterosteus aculeatus |
Heteromorphic homologic XY, GSD, environmental SD | 64 % of homology among X and Y specific contigs Multiple duplications and insertions, insertions of transposons and other repeated sequences on Y chromosome |
10 MYd | Carvalho et al. (2009) |
| Amphibia Family Hylidae Hyla arborea, H. intermedia, and H. molleri |
Homomorphic homologic XY or WZ, TSD | Degeneration of Y chromosome is prevented by rare recombination with X in phenotypic females Y or W chromosome are evolutionary stable (‘fountain-of-youth’ hypothesis) |
5.4–7.1 MYd | Stöck et al. (2011) |
| Amphibia Family Bufonidae Bufo siculus, B. balearicus, B. turanensis and B. shaartusiensis |
Homomorphic homologic XY, TSD | Y chromosome degeneration probably prevented by rare male recombination of X and Y. Y chromosome is evolutionary stable (‘fountain-of-youth’ hypothesis) | 3.3 MYd | Stöck et al. (2013) |
| Reptiles Family Viperidae Pygmy rattlesnake Sistrurus miliarius |
Heteromorphic homologic WZ, GSD | 61 W-linked genes and 712 Z-linked genes. Accumulation of repetitive elements on the W chromosome | ≥50 MYc,d | Vicoso et al. (2013) |
| Reptiles Family Colubridae Garter snake Thamnophis elegans |
Heteromorphic homologic WZ, GSD | 29 W-linked genes and 723 Z-linked genes. Accumulation of repetitive elements on the W chromosome | ≥50 MYc,d | Vicoso et al. (2013) |
| Birds | Heteromorphic homologic WZ, GSD or environmental SD | Different lineages represent different stages of W degradation Number of genes on the W chromosome is tens to 100, while on the Z chromosome ~1000 The W chromosome is a degenerate relict of Z and is the same among species. No W turnovers observed |
120 MYc,d | Wright et al. (2012), Graves (2014), Wright et al. (2014) |
| Mammals | Heteromorphic, homologic XY, GSD | Y chromosome is more degraded than avian W chromosome On the Y chromosome, a few dozen genes are observed while on X ~1000 genes The Y chromosome is a degenerate relict of X. Rare cases of Y turnover or absence are observed among taxa |
>200 MYc,d | Graves (2006), Veyrunes et al. (2008), Bachtrog (2013), Bellott et al. (2014) |
| Homo sapiens | Heteromorphic, homologic XY, GSD | On the Y chromosome, 86 genes have been observed, while on X 1098 genes No difference in TE percentage (44 vs. 54 %) on X- and Y-linked zinc finger genes |
Five evolutionary strata on the Y chromosomed | Skaletsky et al. (2003), Peichel et al. (2004), Ross et al. (2005), Goto et al. (2009) |
Listed examples of W and Y chromosome estimated age, from different taxa are shown below. The sex determination system and available information on chromosome degeneration are given. The listing includes the relevant literature
SD sex determination, GSD genetic sex determination, TSD temperature sex determination, MY million years
aDifferent ways to obtain W/Y chromosome age since recombination stopped
bDegree of heteromorphism
cAge of the group of species
dX–Y or neo-Y—autosome divergence study [after Charlesworth (2012)]