Abstract
All songbirds studied so far have a germline-restricted chromosome (GRC), which is present in the germ cells and absent in the somatic cells. It shows a wide variation in size, morphology, and genetic content between the songbird species. In this paper, we analyzed GRC behavior in female and male meiosis of the great tit, using immunolocalization of meiotic proteins and FISH with GRC-derived DNA probes. We found that, despite dozens of million years of independent evolution, the great tit GRC displays a striking similarity with the GRCs of two species of martins and two species of estrildid finches examined earlier. It was usually present in two copies in females forming recombining bivalent and in one copy in males forming a condensed heterochromatic body with dotted-like axial elements of the synaptonemal complex. We observed mosaicism for the GRC copy number in the female and male great tit. This indicates that one of the GRC copies might be passively lost during premeiotic germ cell divisions. After the meiotic prophase, the GRC was ejected from most male germ cells. The reverse and interspecies FISH with GRC-specific microdissected DNA probes indicates that GRCs of the great tit, pale martin, and zebra finch differ substantially in their genetic content despite similarities in the meiotic behavior.
Highlights
Programmed DNA elimination has been observed in many species of different taxa (Wang and Davis, 2014)
There is a variation in germline-restricted chromosome (GRC) copy number in three species examined: zebra finch, sand martin, and pale martin
We found the signals of the great tit GRC probe at telomeres of some macroand microbivalents of pale martin
Summary
Programmed DNA elimination has been observed in many species of different taxa (Wang and Davis, 2014). In the male germ cells, it is usually present in one copy. It is heterochromatic, highly enriched in histone H3 trimethylated at lysine 9, and ejected from the nuclei after the meiotic divisions (del Priore et al, 2014). The GRC is usually present in two copies, which pair and recombine with each other and are transmitted to the progeny (Pigozzi and Solari, 2005; Torgasheva et al, 2019; Malinovskaya et al, 2020). There is a variation in GRC copy number in three species examined: zebra finch, sand martin, and pale martin. Most oocytes of zebra finches and martins contained two copies of GRC, but some
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