Genetic variation in flowering time induces phenological assortative mating: quantitative genetic methods applied to Brassica rapa

Abstract

It has been argued from first principles that plants mate assortatively by flowering time. However, there have been very few studies of phenological assortative mating, perhaps because current methods to infer paternal phenotype are difficult to apply to natural populations. Two methods are presented to estimate the phenotypic correlation between mates-the quantitative genetic metric for assortative mating-for phenological traits. The first method uses individual flowering schedules to estimate mating probabilities for every potential pairing in a sample. These probabilities are then incorporated into a weighted phenotypic correlation between all potential mates and thus yield a prospective estimate based on mating opportunities. The correlation between mates can also be estimated retrospectively by comparing the regression of offspring phenotype over one parent, which is inflated by assortative mating, to the regression over mid-parent, which is not. In a demonstration experiment with Brassica rapa, the prospective correlation between flowering times (days from germination to anthesis) of pollen recipients and their potential donors was 0.58. The retrospective estimate of this correlation strongly agreed with the prospective estimate. The prospective method is easily employed in field studies that explore the effect of phenological assortative mating on selection response and population differentiation.

Estimating selection through male fitness: three complementary methods illuminate the nature and causes of selection on flowering time

“…First, maternal effects could have delayed AAF in the offspring generation, biasing the estimated response to selection in the experimental evolution method. In B. rapa, the first seeds produced by a dam develop into plants that flower approximately 0.5 days earlier than the last seeds [15]. Assuming this within-plant effect is attributable to a within-plant decline in maternal investment [38], it is possible that among-generation variation in investment per seed similarly affects AAF.…”

“…Summing seed mass sired over intervals yields the individual's expected male fitness. Building on formulae presented in [15,30], we used the flower and seed production schedules to estimate a matrix F ¼ MF, in which each cell F [ j,k] is the expected total mass of seeds with j as dam and k as sire. M represents seed production by maternal plants, with M [ j,d ] equal to seed mass produced by mother j during tagging interval d. F is a K Â D matrix of the fraction of total flowers produced by father k during interval d, with each column summing to 1.…”

“…Inherent to these calculations is the assumption that all flowers dispense the same amount of pollen, which is then randomly vectored to all available stigmas: the distribution of male and female reproductive effort in a population is the sole determinant of an individual's male fitness. Empirical support for this convenient and common [14][15][16][17][18] assumption is mixed. Some support is provided by genetic paternity analyses detecting increased male fitness among individuals whose peak flower production coincides with peak seed production in a population [19,20].…”

Toward a synthetic understanding of the role of phenology in ecology and evolution

“…Adaptive change is facilitated when the trait under selection is also the trait according to which individuals choose mates (Doebeli & Dieckmann 2000;but see Fox 2003). For a trait such as timing of breeding, some level of assortative mating between individuals with similar trait values is inevitable (Fox 2003;Weis & Kossler 2004;Weis 2005), and the resulting inflation of genetic variance can hasten evolutionary change in breeding time (Hendry & Day 2005;Devaux & Lande 2008).…”

Incipient allochronic speciation due to non-selective assortative mating by flowering time, mutation and genetic drift

“…Other models of the origin of reproductive isolation by non-random mating, primarily based on one or two loci, are reviewed by Gavrilets (2004). Weis & Kossler (2004) and Weis (2005) developed methods for estimating the phenotypic correlation between mates for flowering time, accounting for the distribution of individual and population flowering phenology, and Weis et al (2005) suggested the need for further development of multigenic models for the evolution of flowering phenology, which is the purpose of the present paper.…”