Fertility Cost (or Sometimes a Lack of It) in Relation to Heterozygosity for Robertsonian Rearrangements in Mammals: A Review.

Abstract

Background: Robertsonian (Rb) chromosomal rearrangements are very common in mammals and are the primary basis of chromosome number variation between species. The fertility of heterozygotes has particular significance in understanding the mode of fixation of Rb rearrangements, and could have a role in the attainment of reproductive isolation by chromosomally differentiated species.

Summary: Here we survey available data on fertility of Rb heterozygotes in mammals, comparing with homozygotes, and considering effects on litter size, frequencies of anaphase I nondisjunction, germ cell death and pachytene features associated with that germ cell death. We consider both simple heterozygotes which form trivalent configurations at meiosis I and complex heterozygotes which form longer configurations due to heterozygosity for different chromosomes with monobrachial homology. Two species have a particularly wide variety of Rb heterozygotes and have been well studied: the house mouse (the western subspecies) and the common shrew. The overall data confirm that heterozygosity for a single Rb metacentric may be associated with near-normal fertility in mammals, though not in every instance. Usually infertility is not going to be a substantial hindrance to fixation of Rb fusions or fissions. Nor is infertility in simple heterozygotes for one or a few Rb metacentrics on its own likely to promote reproductive isolation. However, simple heterozygotes forming many meiotic trivalents and complex heterozygotes forming long meiotic configurations may suffer substantial infertility or sterility. Even so, heterozygous house mice and common shrews forming the very longest meiotic chains and rings may produce some young. We discuss the implications of these findings with regards the role of Rb rearrangements in speciation.

Key messages: Infertility due to Rb heterozygosity on its own may rarely hinder fixation of Rb rearrangements nor be sufficient to cause a complete interruption to gene flow between hybridizing chromosomal forms. However, this does not rule out a role for Rb rearrangements in speciation. Reinforcement is possible, and Rb rearrangements have the potential to act in synergy with genic incompatibilities to promote reproductive isolation. There can also be the contrary process of despeciation. Natural selection may respond in various ways to a given degree of infertility.