Franco Spirito

The exact values of the probability of fixation of underdominant chromosomal rearrangements

A numerical analysis of the probability of fixation of a chromosomal mutation with partial sterility of the heterozygote in a single population is performed. Three different genetic models are considered: the first model entails constant selection against the heterozygote and is the model almost universally used in previous works; in the other two models selection against the heterozygote depends on its frequency. The exact values of the fixation probability are found by iterating transition matrices with genotype specification. Differences in results among models are small. The exact values found in the first model are compared to estimates obtained from approximations. Solutions based on diffusion models give good approximations when selection against the heterozygote is low, especially if the population is very small. For the higher values of the selection coefficient against the heterozygote, the estimates are rather imprecise, especially when the populations are not very small.

Aneuploidy in metaphases II of spermatocytes of wild house mice from a hybrid zone between a Robertsonian population (CD: 2n = 22) and a population with the standard karyotype (2n = 40)

Meiotic metaphases II from archival slides were studied of male house mice caught in a hybrid zone between a population monomorphic for nine centric fusions (2n = 22) and a population with the standard karyotype (2n = 40), near Rome. The frequency of aneuploidy increases, up to 50%, with increasing number of heterozygous centric fusions (1–4).

Populational interactions among underdominant chromosome rearrangements help them to persist in small demes

Populational interactions among unlinked chromosomal rearrangements with partial heterozygote sterility and multiplicative fitness were studied to verify whether they help such rearrangements to persist in small populations, in spite of a considerable migration rate with a large population. A deterministic island‐continent model was studied by exact recurrence relations connecting gametic frequencies in successive generations.

Reduction of neutral gene flow due to the partial sterility of heterozygotes for a linked chromosome mutation

The effect of linkage between a chromosome mutation producing partially sterile heterozygotes and a neutral locus in reducing the gene flow at the neutral locus is studied using a two-population deterministic model. Chromosome mutations are more efficient in reducing gene flow with low migration rates than with high ones. The interaction between high values of partial heterozygote sterility and low recombination rates can produce, in the low migration pattern, a drastic reduction of gene flow. Nevertheless, since only chromosome mutations with low values of partial heterozygote sterility are likely to be involved in chromosomal speciation, a significant reduction of gene flow will probably occur only for a very limited part of the genome. Therefore, a single chromosome mutation is unlikely to play a primary role in speciation.

Nonrandom segregation in multiple Robertsonian heterozygotes in the house mouse (Mus musculus domesticus): Analysis of metaphases of spermatocytes II of mice from a hybrid zone between a Robertsonian population and a population with the standard karyotype

Abstract Meiotic metaphases II from archival slides were studied in male house mice caught near Rome in a hybrid zone between a population with nine monomorphic centric fusions‐ (CD: 2n = 22) and one with the standard karyotype (2n = 40). The segregation pattern was investigated: it was found that chromosomes of the same form tended to cosegregate.

The reduction of neutral gene flow caused by a selected gene in plant population models.

Abstract The neutral gene flow among populations of higher plants in the presence of a simple zygotic isolating mechanism is studied theoretically. Partial reproductive isolation is caused by a gene subject to selection. The population model is an island model with very small interpopulation migration rates. General solutions have been obtained and verified by numerical tests. It has been observed that the reduction of gene flow caused by the isolating mechanism is stronger—other things being equal—for: (1) greater intensity of selection; (2) lower values of the frequency of recombination between the selected locus and the neutral locus; (3) higher values of the level of partial self-fertilization. The last result is particularly interesting and needs to be considered in discussing speciation processes in plants with high rates of self-fertilization.

Neutral gene flow in the presence of a selected gene with random or assortative mating

Abstract Neutral gene flow in an island model in the presence of a gene subject to selection has been analysed. The selection regime on the gene is such that the maintenance of a stable interpopulation differentiation at this locus is allowed for very low values of the migration rate. Mating at this locus may be random or assortative. Many models of zygotic or prezygotic isolating mechanisms with unifactorial inheritance are represented by this genetic model. Two general solutions for migration rates tending toward zero have been obtained for the case of migration preceding selection and vice versa. It is found that the zygotic isolating mechanisms and one type of prezygotic isolating mechanisms have the same qualitative and quantitative effects for the same values of selection coefficients and of recombination frequencies between the selected gene and the neutral gene. A second type of prezygotic mechanism behaves in a different way: the reduction of gene flow caused by such mechanisms is also a function of the “mating coefficients” of the various genotypes.

Conditions for the persistence of partial zygotic reproductive isolation in an island-continent model

Abstract A theoretical study has been made of the contact between two populations (island-continent model) which are partially isolated insofar as they are monomorphic for different alleles at one or more strictly underdominant loci with multiplicative heterozygote fitnesses. This genetic model of an isolating mechanism is the simplest model of zygotic isolation and is also a very good representation of underdominant chromosomal rearrangements. The particular problem tackled is how the threshold value of the migration rate ( m c ), below which differentiation at the underdominant loci can persist, varies as a function of the number of underdominant loci involved ( n ), for a given level of selection against hybrids (α). It has been found that the stability of genetic differentiation causing a given level α of reproductive isolation decreases as the number of loci involved increases. This effect is particularly noticeable for low levels of isolation. These results have some relevance with regard to the question of whether the establishment of zygotic reproductive isolation between populations occurs more probably as a result of many mutations with small effects on fitness or of only a few mutations with significant effects.