To find any appreciable microsatellite polymorphism in the Raso lark is rather unexpected, given the long-term small size of the population. That this preserved polymorphism is almost exclusively sex-linked is surprising for two reasons. First, none of these markers are reported to be sex-linked in the five species (Table 2) from which they were originally developed. This was confirmed by writing to the authors, asking them to re-test for evidence of sex-linkage; all replied in the negative. In addition, Ase9, Ase18 and Cuu4 have been mapped to chicken autosomes three, three and five, respectively (Dawson et al. 2006), confirming directly the lack of sex-linkage. Second, sex-linked loci are relatively rare. Surveying all issues of Molecular Ecology Resources in 2008, we found 413 new markers developed for 22 species of bird of which only 13 were sex-linked. Only a minority of studies explicitly test for sex-linkage, but of those that do, two large studies reveal 11 of 170 markers to be sex-linked (6.5%) (Jaari et al.
2008; Leder et al. 2008), significantly fewer (χ
= 39.9, using Haber’s correction, P < 0.001), than the six of seven we observed in Raso larks.
Our data further contrast with most other avian studies, which report sex-linked alleles on the Z chromosome only (Ellegren 2000), with all females being phenotypically homozygous (= hemizygous). This reflects the generally much larger size of the Z relative to the W chromosome (75 Mb: 0.26 Mb in chickens) (Dawson et al. 2006). W-linked loci are rare, though appear occasionally (Küpper et al. 2007). Our data reveal a radically different pattern, with, in all but locus Cuμ4, homologous alleles occurring on both the Z and the W chromosomes. By implication, Raso lark sex chromosomes differ markedly from those of other birds. A clue as to what has happened is given by Bulatova (1973), who observed enlarged sex chromosomes in three species of lark, suggestive of either a fusion or a translocation of chromosomal material between the sex chromosomes and one or more autosomes. The three species, bimaculated lark Melanocorypha bimaculata, greater short-toed lark Calandrella brachydactyla (cinerea) and shore lark Eremophila alpestris, all have much larger geographic ranges than the Raso lark. Moreover, Ase18 has been genotyped in skylarks Alauda arvensis (Hutchinson and Griffith 2008), where our re-analysis of the raw data reveals it is sex-linked, and in sparrows (S. Griffith, unpublished data), where we find it is autosomal.
With enlarged sex chromosomes, multiple markers showing a switch from autosomal to sex-linked patterns and allele distributions indicating homologous W and Z alleles, larks, and the Raso lark in particular, appear to have experienced wholesale transfer of material from the autosomes to the sex chromosomes. This transfer appears to have been recent, evidenced both by the unusually large sex chromosomes of some but not all larks, and by the presence of alleles on both the W and Z chromosomes. In most birds, the W chromosome is small, making sex-linked loci hemizygous and reflecting a frequent evolutionary pattern where one of the sex chromosomes tends to degenerate by mutation and loss of material (Charlesworth 1991; Rice 1994). Larks might be unusual in not suffering a degeneration of the W chromosome analogous to the widespread degeneration of the Y chromosome often seen in other animals (Charlesworth and Charlesworth 2000), and also in experimental studies where autosomal material deliberately fused to the sex chromosomes suffers degeneration (Bachtrog and Charlesworth 2000). However, it seems more likely to us that material from a recent autosomal translocation or fusion has had too little time for significant loss to occur. Since three of the six sex-linked loci could be mapped in the chicken genome, we can be confident that at least two autosomes are involved, not just one (an unlikely alternative is that two autosomes fused before a translocation to the sex chromosomes). Just why this has occurred remains unclear. The phenomenon of addition of autosomal material to the sex chromosomes, not dissimilar to that reported in humans (Lahn and Page 1999), appears to be shared by several species of lark including species with large ranges and population sizes (Bulatova 1973).
While the reasons for the transfer are unresolved, we note that it might plausibly provide a mechanism by which some level of heterozygosity could be maintained, even in the face of persistently small population sizes, the two sex chromosomes maintaining two lineages for all markers that reside on them. Small populations usually harbour little variability due to genetic drift, and this is particularly so for the sex chromosomes which, in the absence of strong diversifying selection, are expected to have smaller effective sizes than the autosomes. However, even though each of the sex chromosomes would tend quickly to become monomorphic in a small population, because the W and Z carry different copies, overall greater diversity would be maintained, at least until the W chromosome degeneration began in earnest. This remarkable observation of conserved sex-linked microsatellite diversity in the Raso lark highlights how rapidly genomic reorganisation can occur and provides a fascinating opportunity to study the evolution of autosomal material that has very recently come to lie on a well-established pair of sex chromosomes.