Contact zone between chromosomal races of Mus musculus domesticus. 2. Fertility and segregation in laboratory-reared and wild mice heterozygous for multiple Robertsonian rearrangements

Abstract

Litter size, anaphase I nondisjunction and X±Y dissociation at metaphase I were studied in homozygous and heterozygous house mice from a central Italian chromosomal hybrid zone between the CD (2n 22) race and the standard race (2n 40). We also observed the segregation of the two chromosomal forms (Robertsonian and non-Robertsonian) in male and female multiple heterozygotes from the karyotype of their ospring and chromosomal arm counts of metaphase II. Litter size was signi®cantly reduced in the F1 hybrids, but there was no dierence in litter size between male and female F1s. Fertility in wild mice decreased with increasing numbers of structural heterozygosities (0±5). Some metacentrics appear to be under meiotic drive but there was no rule as to which of the two forms was favoured in backcrosses. An original observation of a negative correlation between the length of metacentrics and transmission rate was described in hybrids. Slight cosegregation of chromosomes with a similar morphology was present in the progeny of males and females. These observations are discussed in relation to the stability of this hybrid zone through time.

Fertility assessment in hybrids between monobrachially homologous Rb races of the house mouse from the island of Madeira: implications for modes of chromosomal evolution

“…Male homozygotes and single trivalentcarrying heterozygotes showed very low levels of aneuploidy and GCD in accordance with data reported for wild house mice elsewhere (Hauffe and Searle, 1998;Castiglia and Capanna, 2000). In contrast, the presence of a chain-of-four meiotic pairing configuration (that is, complex heterozygosity) reduced the reproductive performance of male hybrids, due to both a higher aneuploidy rate and spermatogenetic dysfunction.…”

“…Premeiotic perturbations have been also observed in some cases causing a reduction in germ cell number (Garagna et al, 1990;Hauffe and Searle, 1998;Banaszek et al, 2000;Wallace et al, 2002). Data from different mammalian species (including human) have shown that values of underdominance vary according to the nature of the rearrangement, the chromosomes concerned, the breakpoint site, the gender and also the genetic context and age (King, 1993;Searle, 1993;Djelati et al, 1997;Hauffe and Searle, 1998;Castiglia and Capanna, 2000;Wallace et al, 2002;Pellestor et al, 2005;Anton et al, 2006). Most experimental analyses of the effects of chromosomal heterozygosity on reproductive fitness stem from studies on humans, the house mouse and the common shrew.…”

“…Heterozygotes for fusions produce trivalents at meiosis, formed by the pairing of each metacentric and its two acrocentric homologs. These meiotic configurations usually result in very low nondisjunction (NDJ) rates when few in number, but the degree of gametogenetic impairment increases when many are present (Garagna et al, 1990;Hauffe and Searle, 1998;Castiglia and Capanna, 2000;Wallace et al, 2002). In some instances, however, parapatric Rb races carry monobrachially homologous fusions; that is, Rb metacentrics that have one chromosome arm in common.…”

Contact zones between chromosomal races of Mus musculus domesticus. 3. Molecular and chromosomal evidence of restricted gene flow between the CD race (2n = 22) and the ACR race (2n = 24)

“…All these studies have found hybrids showing trivalents at diakinesis, with hybrid zone widths from 0.5 km (Chatti et al, 1999) to 30 km . Fertility studies show that gene flow should occur freely between races differing in only a few independent metacentrics: heterozygotes for 1-3 metacentrics have normal levels of fertility (Winking et al, 1988;Wallace et al, 1992;Castiglia and Capanna, 2000). However, if the differences between races involve many independent metacentrics the fertility of structural heterozygotes is more severely impaired (Redi and Capanna, 1988;Said et al, 1993;Castiglia and Capanna, 2000).…”

“…Fertility studies show that gene flow should occur freely between races differing in only a few independent metacentrics: heterozygotes for 1-3 metacentrics have normal levels of fertility (Winking et al, 1988;Wallace et al, 1992;Castiglia and Capanna, 2000). However, if the differences between races involve many independent metacentrics the fertility of structural heterozygotes is more severely impaired (Redi and Capanna, 1988;Said et al, 1993;Castiglia and Capanna, 2000). In these extreme cases, both gene flow and the number of hybrid individuals is expected to be low, as in the Tunisian hybrid zone between the Monastir race 2n = 22 and the standard one (Chatti et al, 1999).…”