On the logic of Fisherian sexual selection

In Fisher’s model of sexual selection, a female preference for a male trait spreads together with the trait because their genetic bases become correlated. This can be interpreted as a ‘greenbeard’ system: a preference gene, by inducing a female to mate with a trait-bearing male, favors itself because the male is disproportionately likely also to carry the preference gene. Here, we use this logic to argue that Fisherian sexual selection in diploids proceeds via two channels, corresponding to two reasons that trait-bearing males disproportionately carry preference genes: (i) trait-bearing males are disproportionately the product of matings between preference-bearing mothers and trait-bearing fathers, and thus trait and preference genes are correlated ‘in trans’; (ii) trait and preference genes come into gametic phase disequilibrium, and thus are correlated ‘in cis’. Gametic phase disequilibrium is generated by three distinct mechanisms: a ‘recombination mechanism’, a ‘dominance mechanism’, and a ‘sexual admixture mechanism’. The trans channel does not operate when sexual selection is restricted to the haploid phase, and therefore represents a fundamental difference between haploid and diploid models of sexual selection. We use simulation experiments to artificially eliminate the cis channel, and show that a preference gene can spread in its absence in the diploid model, but not in the haploid model. We further show that the cis and trans channels contribute equally to the spread of the preference when recombination between the preference and trait loci is free, but that the trans channel becomes substantially more important when linkage is tight.

Number of recombinations and genetic properties of a maize population undergoing recurrent selection

In maize recurrent selection programs, selected genotypes were recombined once to generate genetic variability for the next selection cycle. Selection generates negative gametic phase disequilibrium which reduces genetic variances, and this disequilibrium is not significantly reduced with only one generation of recombination. The objective of this research was to assess the effects of one additional generation of recombination on phenotypic and genotypic parameters in a maize population undergoing recurrent selection. Selected progenies of the EPB-4 population were subjected to one and two generations of recombination, and from each generation half- and full-sib progenies were developed and evaluated at three environments for grain yield, plant and ear heights, prolificacy, and ear placement. There were no significant changes between each progeny type with one and two generations of recombination for the means, ranges, phenotypic distribution of the traits, genetic variances, heritability coefficients, and genetic correlations for the traits assessed. The results suggest that an additional generation of recombination will not increase the effectiveness of maize recurrent selection programs.

Genetic Structure of Setosphaeria turcica Populations in Tropical and Temperate Climates

Northern leaf blight, caused by Setosphaeria turcica, is a serious disease of maize in temperate and tropical environments. To examine the pathogen's population structure, we analyzed 264 isolates from four different continents with 70 random amplified polymorphic DNA markers and determined their mating types. Tropical populations (from Kenya, Mexico, and southern China) had an extremely high genotypic diversity, no or only weak gametic phase disequilibrium, and an even distribution of the two mating types, indicating frequent sexual recombination. Temperate populations (from Europe and northern China) had a much lower genotypic diversity, strong gametic phase disequilibrium, and an uneven distribution of mating types, indicating that sexual recombination has been rare. Populations in different continents were genetically isolated. They shared no haplotypes and carried several “private” alleles. The number of migrants between continents and between regions (between northern and southern China, western and central Kenya, and Europe west and east of the Alps) was estimated to be less than one per generation. Multivariate statistics suggested a greater relatedness of populations from the same continents than from different continents. Within agroecological zones, migration must be extensive. The potential within populations of S. turcica for adaptation should be regarded as very high, especially in tropical climates.