Recombination Map of the Common Shrew,Sorex araneus(Eulipotyphla, Mammalia)

Abstract

The Eurasian common shrew (Sorex araneus L.) is characterized by spectacular chromosomal variation, both autosomal variation of the Robertsonian type and an XX/XY 1 Y 2 system of sex determination. It is an important mammalian model of chromosomal and genome evolution as it is one of the few species with a complete genome sequence. Here we generate a high-precision cytological recombination map for the species, the third such map produced in mammals, following those for humans and house mice. We prepared synaptonemal complex (SC) spreads of meiotic chromosomes from 638 spermatocytes of 22 males of nine different Robertsonian karyotypes, identifying each autosome arm by differential DAPI staining. Altogether we mapped 13,983 recombination sites along 7095 individual autosomes, using immunolocalization of MLH1, a mismatch repair protein marking recombination sites. We estimated the total recombination length of the shrew genome as 1145 cM. The majority of bivalents showed a high recombination frequency near the telomeres and a low frequency near the centromeres. The distances between MLH1 foci were consistent with crossover interference both within chromosome arms and across the centromere in metacentric bivalents. The pattern of recombination along a chromosome arm was a function of its length, interference, and centromere and telomere effects. The specific DNA sequence must also be important because chromosome arms of the same length differed substantially in their recombination pattern. These features of recombination show great similarity with humans and mice and suggest generality among mammals. However, contrary to a widespread perception, the metacentric bivalent tu usually lacked an MLH1 focus on one of its chromosome arms, arguing against a minimum requirement of one chiasma per chromosome arm for correct segregation. With regard to autosomal chromosomal variation, the chromosomes showing Robertsonian polymorphism display MLH1 foci that become increasingly distal when comparing acrocentric homozygotes, heterozygotes, and metacentric homozygotes. Within the sex trivalent XY 1 Y 2 , the autosomal part of the complex behaves similarly to other autosomes.

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

“…The distribution of MLH1 foci along the SCs of the Siberian shrews matched the pattern described for the shrews sampled from Oxford-Wirral hybrid zone [18]. For all chromosome arms examined we observed pronounced peaks at the distal regions and a deficiency of the foci in pericentromeric regions (Figures 7-8).…”

“…The sex trivalent X/Y1Y2 followed its usual pairing pattern [18,26,27]: the autosomal part of Xq (arm d) and the whole Y2 were completely synapsed, while the Y1 (arm s) was partly paired with the end of Xp (arm e) in a short pseudoautosomal region (Figure 2A). Pachytene cells of simple heterozygotes for one or two CIII usually contained completely synapsed trivalents with or without formation of side arms between pericentromeric regions of the twin acrocentrics.…”

“…To visualise synaptonemal complexes (SC) and the sites of crossing over we used fluorescently-labelled antibodies to SYCP3, a protein of the lateral elements of SC, and MLH1, a mismatch repair protein marking about 90% of mature recombination nodules. This approach has been successfully applied for mapping the sites of recombination in several species of mammals [15][16][17], including the common shrew [18] …”

Reduced recombination patterns in Robertsonian hybrids between chromosomal races of the house mouse: chiasma analyses

“…Although mean values varied between analyses, F1 hybrids/trivalents always exhibited the same distribution pattern: a pronounced decrease in recombination in the proximal regions of the chromosomes compared with St mice/St bivalents. Such modifications in the recombination landscape owing to Rb fusions appear to be a general feature of this type of rearrangement as they have also been observed in a diversity of organisms, that is, shrews, grasshoppers and fish (Borodin et al, 2008;Colombo, 2013;Ostberg et al, 2013). Thus, a growing number of studies demonstrate that heterozygosity for Rb rearrangements is associated with reduced recombination in the proximal regions of chromosomes.…”

Chromosomes and speciation in tuco-tuco (Ctenomys, Hystricognathi, Rodentia)

“…В то же время получены свидетельства в пользу современной версии этой гипотезы. Существенные изменения в реком-бинации наблюдались у мышей и землероек -носителей простых и сложных гетерозигот по робертсоновским транслокациям по сравнению с гомозиготами (Searle, 1986(Searle, , 1993Wallace et al, 1992;Bidau et al, 2001;Dumas, BrittonDavidian, 2002;Borodin et al, 2008;Merico et al, 2013;Dumas et al, 2015).…”