Scriven, Paul N; (1993) Robertsonian translocations in British mice: an experimental study. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The processes by which new species are formed have long been open to question. Chromosomal rearrangement has been thought to be important in speciation. Central to the idea that chromosome rearrangements have a role in speciation, is that heterozygotes between chromosome races have reduced fertility. A major problem with chromosome speciation models is that if a rearrangement is effective as an isolating mechanism, it must also have a low probability of being established in a population. This study describes the artificial creation of a house mouse Robertsonian polymorphism on a small island, and the possibility that centric fusions are isolating is investigated. House mice from the Orkney island of Eday (three centric fusions, 2n=34) were released by R.J. Berry and his associates on to the Isle of May, Firth of Forth (standard house mouse karyotype, 2n=40). Within 18 months of introduction each centric fusion had increased in frequency from an estimated starting value of 8% to a value close to 50%, and apparently stabilised three years later with values around 65% for all three fusions, each segregating in accordance with Hardy-Weinberg expectations. The transformed population was behaving as a panmictic unit. Male Eday-May F1 hybrids were found to have a relatively low frequency of non-disjunction (13%). Objective data are provided by morphometric analysis of the mandible. Significant changes due to inherited factors were found after a very few generations. Very good agreement was found between single gene loci (represented by allozyme data) and morphology of the mandible. Principal component analysis clearly resolved size and shape differences between the samples and showed post-introduction mice were significantly larger than Eday and pre-introduction May mice, and intermediate in shape. The size difference is a multivariate indication of heterosis (6-8%). 54% of the total post-introduction mandible variation was due to genetic differences. A hybrid index using six blood allozymes (devised to investigate introgression at the level of individual mice), Nei's genetic distance calculated with 19 allozyme loci, and mandible shape, placed post-introduction May mice mid-way between Eday and pre-introduction May. Post-introduction mice were more variable electrophoretically and morphologically than pre-introduction May mice. The establishment that centric fusions are not strongly underdominant (from the stabilization of a Robertsonian polymorphism, non-significant deviations from Hardy-Weinberg equilibrium, and relatively low rates of nondisjunction) , means that factors such as deme size, vagility, meiotic drive, inbreeding, and genetic drift, can be revised, and the reality of chromosomal speciation discounted.

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