Chromosome Synapsis and Recombination in Male Hybrids Between Two Chromosome Races of the Common Shrew (Sorex araneus L., Soricidae, Eulipotyphla)

The hybrid zone between chromosome races of the common shrew (Sorex araneus) provides an exceptional model to study the potential role of chromosome rearrangements in the initial steps of speciation. The Novosibirsk and Tomsk races differ by a series of Robertsonian fusions with monobrachial homology. They form a narrow hybrid zone and generate hybrids with both simple (chain of three chromosomes) and complex (chain of eight or nine) synaptic configurations. Using immunolocalisation of the meiotic proteins, we examined chromosome pairing and recombination in males from the hybrid zone. Homozygotes and simple heterozygotes for Robertsonian fusions showed a low frequency of synaptic aberrations (< 10%). The carriers of complex synaptic configurations showed multiple pairing abnormalities, which might lead to reduced fertility. Recombination frequency in the proximal regions of most chromosomes of all karyotypes was much lower than in the other regions. The strong suppression of recombination in the pericentromeric regions and co-segregation of race specific chromosomes involved in the long chains would be expected to lead to linkage disequilibrium between genes located there. Genic differentiation, together with the high frequency of pairing aberrations in male carriers of the long chains, might contribute to maintenance of the narrow hybrid zone.

Chromosome pairing and recombination in male hybrids between two chromosome races of the common shrew (Sorex araneus L., Soricidae, Eulipotyphla)

The hybrid zone between chromosome races of the common shrew (Sorex araneus) provides an exceptional model to study the potential role of chromosome rearrangements in the initial steps of speciation. The Novosibirsk and Tomsk races differ by a series of Robertsonian fusions with monobrachial homology. They form a narrow hybrid zone and generate hybrids with both simple (chain of three chromosomes) and complex (chain of eight or nine) synaptic configurations. Using immunolocalisation of the meiotic proteins, we examined chromosome pairing and recombination in males from the hybrid zone. Homozygotes and simple heterozygotes for Robertsonian fusions showed a low frequency of synaptic aberrations (< 10%). The carriers of complex synaptic configurations showed multiple pairing abnormalities, which might lead to reduced fertility. Recombination frequency in the proximal regions of most chromosomes of all karyotypes was much lower than in the other regions. The strong suppression of recombination in the pericentromeric regions and co-segregation of race specific chromosomes involved in the long chains would be expected to lead to linkage disequilibrium between genes located there. Genic differentiation, together with the high frequency of pairing aberrations in male carriers of the long chains, might contribute to maintenance of the narrow hybrid zone.

Estudios cromosómicos en seis especies mexicanas de comelináceas

In the present study, chromosome numbers (2n) and karyotypes for the following taxa were obtained: Tradescantia crassifolia Cav. subsp. acaulis (Martens y Galeotti) D. Hunt, Gibasis shiedeana (Kunth) D. Hunt, Cymbispatha commelinoides (Shult. f.) Pichon, Commelina communis L., C. diffusa Burm. f. and C. coelestis Willd. For T. crassifolia subsp. acaulis, this is the first report on chromosome number and karyotype. The species studied were collected in the Reserva Ecológica del Pedregal de San Ángel, México, D.F. The differences encountered in chromosome numbers and karyotypes are discussed. Our results provide evidence that Robertsonian fusions and fisions, structural rearrangements, polyploidy and aneuploidy have an important role in the speciation of Commelinaceae members.

Gene flow despite complex Robertsonian fusions among rock-wallaby ( Petrogale ) species

Complex Robertsonian rearrangements, with shared arms in different fusions, are expected to prevent gene flow between hybrids through missegregation during meiosis. Here, we estimate gene flow between recently diverged and chromosomally diverse rock-wallabies ( Petrogale ) to test for this form of chromosomal speciation. Contrary to expectations, we observe relatively high admixture among species with complex fusions. Our results reinforce the need to consider alternative roles of chromosome change, together with genic divergence, in driving speciation.