Karyotypes and recombination patterns of the Common Swift (Apus apus Linnaeus, 1758) and Eurasian Hobby (Falco subbuteo Linnaeus, 1758)

Background Meiotic recombination is an important source of genetic variability. Studies on mammals demonstrate a substantial interspecies variation in overall recombination rate, which is dependent mainly on chromosome (2n) and chromosome arm number (FN). Bird karyotypes are very conservative with 2n being about 78-82 and FN being 80-90 in most species. However, some families such as Apodidae (swifts) and Falconidae (falcons) show a substantial karyotypic variation. In this study, we describe the somatic and pachytene karyotypes of the male Common Swift (Apus apus) and the pachytene karyotype of the male Eurasian Hobby (Falco subbuteo) and estimate the overall number and distribution of recombination events along the chromosomes of these species.

Methods The somatic karyotype was examined in bone marrow cells. Pachytene chromosome spreads were prepared from spermatocytes of adult males. Synaptonemal complexes and mature recombination nodules were visualized with antibodies to SYCP3 and MLH1 proteins correspondingly.

Results The karyotype of the Common Swift consists of three metacentric, three submetacentric and two telocentric macrochromosomes and 31 telocentric microchromosomes (2n = 78; FN = 90). It differs from the karyotypes of related Apodidae species described previously. The karyotype of the Eurasian Hobby contains one metacentric and 13 telocentric macrochromosomes and one metacentric and ten telocentric microchromosomes (2n = 50; FN = 54) and is similar to that described previously in 2n, but differs for macrochromosome morphology. Despite an about 40% difference in 2n and FN, these species have almost the same number of recombination nodules per genome: 51.4 ± 4.3 in the swift and 51.1 ± 6.7 in the hobby. The distribution of the recombination nodules along the macrochromosomes was extremely polarized in the Common Swift and was rather even in the Eurasian Hobby.

Conclusions This study adds two more species to the short list of birds in which the number and distribution of recombination nodules have been examined. Our data confirm that recombination rate in birds is substantially higher than that in mammals, but shows rather a low interspecies variability. Even a substantial reduction in chromosome number does not lead to any substantial decrease in recombination rate. More data from different taxa are required to draw statistically supported conclusions about the evolution of recombination in birds.