Abstract
All songbirds studied to date have an additional Germline Restricted Chromosome (GRC), which is not present in somatic cells. GRCs show a wide variation in genetic content and little homology between species. To check how this divergence affected the meiotic behavior of the GRC, we examined synapsis, recombination and copy number variation for GRCs in the closely related sand and pale martins (Riparia riparia and R. diluta) in comparison with distantly related estrildid finches. Using immunolocalization of meiotic proteins and FISH with GRC-specific DNA probes, we found a striking similarity in the meiotic behavior of GRCs between martins and estrildid finches despite the millions of years of independent evolution. GRCs are usually present in two copies in female and in one copy in male pachytene cells. However, we detected polymorphism in female and mosaicism in male martins for the number of GRCs. In martin and zebra finch females, two GRCs synapse along their whole length, but recombine predominately at their ends. We suggest that the shared features of the meiotic behavior of GRCs have been supported by natural selection in favor of a preferential segregation of GRCs to the eggs.
Highlights
All songbirds studied to date have an additional Germline Restricted Chromosome (GRC), which is not present in somatic cells
We examine the meiotic behavior and copy number variation of GRCs in the germ cells of pale martin females and males and sand martin females sampled from natural populations
The somatic karyotypes of the sand martin and the pale martin included 39 pairs of autosomes and a pair of sex chromosomes (Fig. 1a), matching the description of the sand martin karyotype given by Li and Bian[10]
Summary
All songbirds studied to date have an additional Germline Restricted Chromosome (GRC), which is not present in somatic cells. We examine the meiotic behavior and copy number variation of GRCs in the germ cells of pale martin females and males and sand martin females sampled from natural populations. Most GRC bivalents in the martin oocytes contained two MLH1 foci. The average number of MLH1 foci per GRC bivalent was the same in sand martin and pale martin oocytes (1.9 ± 0.3 and 1.9 ± 0.4, correspondingly; Mann–Whitney U test; p = 0.731).
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