Comparison of Karyotypes in Two Hybridizing Passerine Species: Conserved Chromosomal Structure but Divergence in Centromeric Repeats

Abstract

Changes in chromosomal structure involving chromosomal rearrangements or copy number variation of specific sequences can play an important role in speciation. Here, we explored the chromosomal structure of two hybridizing passerine species; the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), using conventional cytogenetic approaches, immunostaining of meiotic chromosomes, fluorescence in situ hybridization as well as comparative genomic hybridization (CGH). We found that the two nightingale species show conserved karyotypes with the same diploid chromosome number of 2n = 84. In addition to standard chromosomes, both species possessed a small germline restricted chromosome of similar size as a microchromosome. Just a few subtle changes in chromosome morphology were observed between the species, suggesting that only a limited number of chromosomal rearrangements occurred after the species divergence. The interspecific CGH experiment suggested that the two nightingale species might have diverged in centromeric repetitive sequences in most macro- and microchromosomes. In addition, some chromosomes showed changes in copy number of centromeric repeats between the species. The observation of very similar karyotypes in the two nightingale species is consistent with a generally slow rate of karyotype evolution in birds. The divergence of centromeric sequences between the two species could theoretically cause meiotic drive or reduced fertility in interspecific hybrids. Nevertheless, further studies are needed to evaluate the potential role of chromosomal structural variations in nightingale speciation.

Rapid gene content turnover on the germline-restricted chromosome in songbirds

“…Both species had a GRC comparable in size with a microchromosome (i.e. a micro-GRC) as was described in Poignet et al (2021). Consistent with this, the GRC micronucleus for both nightingale species was much smaller than in species with a macro-GRC (Supplementary Fig.…”

“…Measurements were made of immunostained synaptonemal complexes of pachytene chromosomes from Poignet et al (2021). Chromosomes were immunostained with anti-SYCP3 antibody recognizing the lateral elements of the synaptonemal complex, and human anticentromere serum (CREST, 15-234, Antibodies Incorporated) binding kinetochores (see Poignet et al 2021 for details).…”

“…Measurements were made of immunostained synaptonemal complexes of pachytene chromosomes from Poignet et al (2021). Chromosomes were immunostained with anti-SYCP3 antibody recognizing the lateral elements of the synaptonemal complex, and human anticentromere serum (CREST, 15-234, Antibodies Incorporated) binding kinetochores (see Poignet et al 2021 for details). The GRC can be recognized from other chromosomes on these slides by its relatively weaker staining by anti-SYCP3 antibody and the CREST signal which covers the whole chromosome, instead of just the centromere.…”

“…In this paper, we sequenced and assembled the GRC in two closely related songbird species, the common nightingale (Luscinia megarhynchos) and thrush nightingale (L. luscinia), both of which possess a micro-GRC (Poignet et al, 2021). These species from the Muscicapidae family diverged approximately 1.8 mya (Storchová et al 2010) and still hybridize in a secondary contact zone (Reifová, et al 2011;Mořkovský, et al 2018;Albrecht, et al 2019).…”

Karyotypes of Manatees: New Insights into Hybrid Formation (Trichechus inunguis × Trichechus m. manatus) in the Amazon Estuary

“…In this case, segregation errors can occur between the pairs involved in the multi-chromosomal association of the F1 hybrid, since they are susceptible to a non-alternating distribution of their components [ 37 ]. Other factors that can increase the probability of errors during Anaphase I are variations in the number of copies of repetitive sequences present in the centromere, which can cause segregation problems in gametogenesis in female hybrids, by altering the meiotic drive [ 38 ]. Thus, part of the elements of Vitor’s karyotype may correspond to univalents, which originated through the uneven and random distribution of chromosomes during anaphase I in the progenitor hybrid female (F1); univalents may show anaphase delay, and for this reason, tend to be eliminated more frequently [ 37 ]; in this case, unbalanced gametes would be formed in large proportion, and the probability of reconstituting the 2n = 50 karyotype would be extremely low in the F3 generation after backcrossing.…”