Chromosomal segregation and fertility in Robertsonian chromosomal heterozygotes of the house musk shrew (Suncus murinus, Insectivora, Soricidae)

Karyotypic polymorphism of Crocidura tanakae (Eulipotyphla: Soricidae) and revision of the karyotype of C. attenuata in mainland China

Conventional karyotypes of Crocidura tanakae and C. attenuata collected from mainland China were examined by bone marrow and cell culture methods along with identification to species using molecular data. In contrast to C. tanakae from Taiwan, populations from mainland China were karyotypically polymorphic, varying in diploid number (2n) from 24 to 40 and fundamental number (FN) from 45 to 56. Autosomes are composed of 6–20 metacentric or submetacentric chromosomes, 0–8 subtelocentric chromosomes, and 0–24 telocentric chromosomes. These polymorphic karyotypes indicate the presence of two geographically separated races. The ancestral karyotype of C. tanakae potentially is the same as currently found on Taiwan: 2n = 40 and FN = 56. In contrast to C. tanakae, C. attenuata had a uniform karyotype (2n = 40, FN = 54) in mainland China, suggesting that previously reported karyotypic polymorphism in this species was an error due to the morphological crypsis between C. tanakae and C. attenuata in mainland China. The intricate karyotypic polymorphism of C. tanakae may be indicative of this species undergoing speciation.

Chromosomal polymorphism in mammals: an evolutionary perspective

Although chromosome rearrangements (CRs) are central to studies of genome evolution, our understanding of the evolutionary consequences of the early stages of karyotypic differentiation (i.e. polymorphism), especially the non-meiotic impacts, is surprisingly limited. We review the available data on chromosomal polymorphisms in mammals so as to identify taxa that hold promise for developing a more comprehensive understanding of chromosomal change. In doing so, we address several key questions: (i) to what extent are mammalian karyotypes polymorphic, and what types of rearrangements are principally involved? (ii) Are some mammalian lineages more prone to chromosomal polymorphism than others? More specifically, do (karyotypically) polymorphic mammalian species belong to lineages that are also characterized by past, extensive karyotype repatterning? (iii) How long can chromosomal polymorphisms persist in mammals? We discuss the evolutionary implications of these questions and propose several research avenues that may shed light on the role of chromosome change in the diversification of mammalian populations and species.

Reproductive isolation due to the genetic incompatibilities between Thrichomys pachyurus and two subspecies of Thrichomys apereoides (Rodentia, Echimyidae)

We tested intrinsic reproductive isolation between 3 taxa of the South American caviomorph rodent Thrichomys (Rodentia, Echimyidae): T. pachyurus, T. apereoides subsp. apereoides and T. apereoides subsp. laurentius. They were mated in captivity and produced viable progeny. Some F1 hybrid females were fertile, whereas all F1 males were sterile. Histological examination revealed meiotic arrest at the primary spermatocyte stage. No sperm was detected in testes or epididymes. Electron microscopic analysis of surface spread synaptonemal complexes revealed a complete failure of chromosome pairing in F1 hybrids of T. pachyurus with T. apereoides subsp. laurentius and T. apereoides subsp. apereoides. In the male hybrids between T. apereoides subsp. apereoides and T. apereoides subsp. laurentius, meiosis did not proceed beyond diplotene, although all of the chromosomes, including heteromorphic ones, paired in an orderly fashion. Backcross males with homomorphic karyotypes showed segregation in meiosis progression. This indicates that male hybrid sterility is due to genetic, but not chromosomal, incompatibility of the parental taxa.Key words: hybrid sterility, speciation, chromosome rearrangements, meiosis, spermatogenesis, synaptonemal complex, Thrichomys.

Characterization of a hybrid zone between two chromosomal races of the weta Hemideina thoracica following a geologically recent volcanic eruption

Two chromosomal races (2n=17 and 2n=15; XO) of the weta Hemideina thoracica meet at the centre of a volcanic region in North Island, New Zealand. Five independent polymorphic genetic markers showed broadly coinciding, steep frequency clines from north to south across this zone beside the flooded crater, Lake Taupo. Three unlinked nuclear gene markers provide estimates of zone width that are at least twice the width of the chromosomal and mitochondrial clines, with cline centres displaced at least 2.5 km. The different zone widths and centres suggest that this hybrid zone is a semipermeable barrier reducing the introgression of the chromosomal markers more than genic markers. We estimate that this species of weta must have a dispersal rate of at least 100 m per generation using the time since the last Taupo eruption (1850 years ago), which covered an area of about 20 000 km2 with pyroclastic flow.