Meiosis and speciation: A study in a speciatingMus terricolor complex

The three chromosomal species of theMus terricolor complex possess 2n = 40 chromosomes. We show that their karyotypes differ in stable heterochromatin variations fixed in homozygous condition as prominent short arms in autosomes 1, 3 and 6. The three chromosomal species exhibit a high incidence of polymorphisms for Robertsonian fusions and pericentric inversions. Breeding experiments and histological analysis of testis show that heterozygosity for pericentric inversions and Robertsonian fusions had no effect on fertility. Meiotic analysis shows normal overall progression of meiosis in the heterozygotes, which is consistent with their normal gametogenesis. Nevertheless, both the inversion and fusion heterozygotes had undergone some alterations in the regular process of homologous synapsis, and it appeared that certain features of the meiotic system circumvented the potential negative effects of these polymorphic chromosomal rearrangements. The results indicate that the attributes of the meiotic system in a given organism could modulate the potential of a chromosomal rearrangement as reproductive barrier. The meiotic modulation hypothesis offers an explanation for the contradictory effects of the similar kinds of chromosomal mutations reported in different species.     

Phylogenetic analysis of the distribution of chromosomal races of Mus mnscuius domesticus Rutty in Europe

Robertsonian (Rb) translocation is a common chromosomal rearrangement in the house mouse. In free-living populations, 79 fusions with different combinations of chromosomes 1 to 18 have been found in some 45 populations. An updated list of these fusions is presented and analysed in order to reveal the possible processes by which the fusions spread within or among populations. A widespread hypothesis is that when two populations share the same fusion, it can be assumed that they have a common ancestor. This can serve as the basis for the use of the cladistic methods. While I present such an analysis on the updated list of Rbs, I also point to the problems associated with it in this case because many fusions have multiple origins and exchanges of Rbs between populations are frequent. I have tried to use a different approach, based on a critical and quantitative evaluation of the hypothesis of common ancestry. Assuming that the 153 possible fusions have an equal probability of occurrence, I give the formula to compute the probability that populations share a given number of fusions by chance alone. Only when this probability is lower than a chosen level (say 5%) can the populations be inferred to have a non-independent origin (i.e. they have a common ancestor or they have exchanged chromosomes by introgression). This probability measure is then used as a distance estimate to show the relationship between all the Rb populations. This analysis suggests that although some Rbs must have occurred more than once, most of the populations have non-independent origins. Almost all the populations from northern Africa to Belgium and Germany appear to have close karyotopic relationships and form a major group. Clearly independent Rb populations are mainly found in the periphery of this major group, for example in Scotland, Denmark and Spain. 'Chromosomal' flow between Rb populations appears to be a very important process.     

Cytogenetics of the genus Arvicanthis (Rodentia,Muridae). 3. Comparative cytogenetics of A. neumanni and A. nairobae from Tanzania

The African rats of the genus Arvicanthis have been widely studied during recent years to clarify species boundaries and phylogenetic relationships. The wide chromosomal variability of the genus has been highlighted in several studies, with each accepted species characterised by its individual karyotypes and others being revealed as cryptic species. In the present paper we report the karyotype and the C- and G-banding patterns of the two species A. nairobae and A. neumanni from seven localities of Tanzania, an area of the range poorly studied. The two karyotypes were compared to that of A. niloticus, which is considered to be primitive. The karyotype of A. neumanni is characterised by 2n = 53–54 and NFa = 62. This karyotypic variability depends on a widespread Robertsonian polymorphism. The karyotype of A. nairobae shows 2n = 62 and NFa = 78; it diverges from that of A. niloticus through one reciprocal translocation, five inversions and three heterochromatic additions. The comparison with the karyotypes of other species of the genus showed that A. neumanni belongs to the east African lineage (with A. abyssinicus, A. blicki, A. niloticus), while A. nairobae is closer to the central and the west African representatives which were all previously under the name of A. niloticus (ANI-2, ANI-3, ANI-4). The distribution of A. nairobae in east Africa opens new scenarios in the biogeographical pattern of evolution of the genus.     

A microsatellite linkage map for Drosophila montana shows large variation in recombination rates,and a courtship song trait maps to an area of low recombination

Current advances in genetic analysis are opening up our knowledge of the genetics of species differences, but challenges remain, particularly for out‐bred natural populations. We constructed a microsatellite‐based linkage map for two out‐bred lines of Drosophila montana derived from divergent populations by taking advantage of the Drosophila virilis genome and available cytological maps of both species. Although the placement of markers was quite consistent with cytological predictions, the map indicated large heterogeneity in recombination rates along chromosomes. We also performed a quantitative trait locus (QTL) analysis on a courtship song character (carrier frequency), which differs between populations and is subject to strong sexual selection. Linkage mapping yielded two significant QTLs, which explained 3% and 14% of the variation in carrier frequency, respectively. Interestingly, as in other recent studies of traits which can influence speciation, the strongest QTL mapped to a genomic region partly covered by an inversion polymorphism.     

Experimental evolution with yeast

Many of the difficulties of studying evolution in action can be surmounted using populations of microorganisms, such as yeast. A readily manipulated sexual system and an increasingly sophisticated array of molecular and genomic tools uniquely qualify Saccharomyces cerevisiae as an experimental subject. This minireview briefly describes some recent contributions of yeast experiments to current understanding of the evolution of ploidy, sex, mutation, and speciation.     

GENETIC DIFFERENTIATION OF POLISH POPULATIONS OF SOREX ARANEUS L. III. INTERCHROMOSOMAL RECOMBINATION IN A HYBRID ZONE

Two parapatric chromosomal races of the common shrew (Sorex araneus) in Poland differ in their complement of metacentric arm combinations: hk, io, gr, nm (race IV), and hi, ko, gm, np (race II). In hybrids, these eight race-diagnostic metacentrics form two randomly segregating complexes. The first complex (C₁) occurs in the form of a ring configuration ok/kh/hi/io, or a chain o/ok/kh/hi/i (when there is Robertsonian polymorphism of the element io). The second complex (C₂) always takes the form of a six-element chain configuration r/rg/gm/mn/np/p. The C₂ complex may be shortened to five or even four elements, when acrocentrics g, m and n are present. In the contact zone we found shrews of pure races (race II or IV), as well as hybrids with C₁ or C₂ complexes, and recombinants hi, ko, gr, nm. Complex heterozygotes are likely to suffer reduced fertility due to malsegregation at meiosis. However, the C₁ hybrids with ring configurations occur with a high frequency throughout the contact zone. This suggest that their fitness is only slightly lowered relative to pure race individuals, in contrast to the hybrids with C₁ or C₂ chain configurations, which presumably have a more heavily reduced fertility. On the other hand, at the center of the zone there is a high proportion of recombinants, which, being chromosomal homozygotes, should display normal meiotic segregation. Furthermore, the high frequencies of recombinants within the contact zone should facilitate gene flow between the races. The occurrence of recombinants plays a similar role as the appearance of the maximum frequencies of acrocentric homozygotes described in several contact zones of S. araneus.