Parental Effects and Gender Specialization in a Tropical Heterostylous Shrub

Abstract

Abstract. Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense, thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.

Reproductive ecology of distylous Palicourea padifolia (Rubiaceae) in a tropical montane cloud forest. II. Attracting and rewarding mutualistic and antagonistic visitors

“…Theoretical and empirical studies have stressed the role of an asymmetrical pollen transfer between floral morphs, and consequently of pollinators, as the main evolutionary force in the process of gender specialization in distylous species (Ganders, 1974;Ornduff, 1975;Charlesworth and Charlesworth, 1979;Lloyd, 1979;Beach, 1981;Barrett, 1992, and references therein). Although intrinsic genetic factors may affect gender specialization in plants (Casper, 1992;Domínguez et al, 1997;Á vila-Sakar and Domínguez, 2000), the nature and intensity of other selective pressures favoring gender specialization has rarely been investigated (Olesen, 1979;Contreras and Ornelas, 1999;Leege and Wolfe, 2002). Yet, because cross-pollination is necessary for reproductive success in both morphs, they should not differ in attributes that contribute to attracting or rewarding floral visitors (Leege and Wolfe, 2002), leading to equal reproductive success.…”

Pre-dispersal seed predation reduces the reproductive compensatory advantage of thrum individuals in Erythroxylum havanense (Erythroxylaceae)

“…Previous studies have suggested that E. havanense is experiencing a process of gender specialization in which most thrum plants produce twice as much seeds as pins, whereas most pin individuals produce fewer seeds but a higher proportion of viable pollen (Domínguez et al 1997;Avila-Sakar and Domínguez 2000;Rosas et al 2005). Results from this study indicate that the disproportionate damage inflicted by seed predators on thrum individuals is owing to their greater seed production.…”

“…2), to only 48% of the total seed crop, a 23% disadvantage. However, even when seed predators may consume up to 52% of the total seed crop of large thrum individuals, plants from this morph still have a fecundity advantage over pin individuals (see also Avila-Sakar and Domínguez 2000;Rosas et al 2005).…”

“…Although there are no differences in the number of flowers or the amount of pollen produced by each morph, thrum plants produce at least twice as many seeds, but 51% fewer viable pollen than pin plants (Domínguez et al 1997;Cuevas et al 2005;Rosas et al 2005). The seeds produced by thrum plants also have greater germination rates, a superior vegetative performance, and a greater probability of reproduction (Avila-Sakar and Domínguez 2000). Thus, thrum plants efficiently compensate for the loss of the male function by producing more seeds of greater quality than pin plants.…”

“…The goal of this study was to assess the impact of pre-dispersal seed predation on the compensatory advantage of Erythroxylum havanense, a heterostylous species with morphbiased male-sterility (see below, and Domínguez et al 1997;Avila-Sakar and Domínguez 2000), and to evaluate how flowering phenology influences the probability of seed predation in each morph. Under the assumption that seed predation is density dependent and seed predators concentrate on large-crop plants, we hypothesized that they would reduce the compensatory female advantage of thrum individuals (the more male-sterile morph).…”