Self- and Cross-Fertilization in Plants. I. Functional Dimensions

Abstract

Many functional-ecological, morphological, and physiological-factors affect the occurrence of selffertilization. Six modes of self-pollination are distinguished. These differ in whether they utilize specialized flowers, whether they involve the transfer of pollen within or between flowers, whether they are autonomous or mediated by vectors, and their timing relative to opportunities for outcrossing. The various modes of selfing are subject to different structural constraints. Prepotency, the preferential success of cross-pollen in achieving fertilizations when it competes with self-pollen, influences the frequency of selffertilization in some species. The amount of self-fertilization may depend on environmental conditions and the vector species visiting each flower and may vary among the flowers of one plant. To gain information on the prevalence of autonomous self-pollination, 66 species for which the degrees of selfcompatibility and autofertility (seed set in isolation) have been published were surveyed. Partially selfincompatible species (in which the seed set is lower after self-pollination than after separate outcrosses) have on average lower autofertility than self-compatible species (in which self-and cross-pollinations succeed equally well), but some partially self-incompatible species have considerable autofertility and some self-compatible species have none. A number of features of floral morphology and phenology are associated with high Autofertility Indices. Introduction The comparison of self-and cross-fertilization is the central topic of floral biology. Following the discovery by Knight (1799) and Darwin (1868, 1876) that cross-fertilization is advantageous because it produces superior progeny, there was a period of intense activity in pollination biology in the later decades of the nineteenth century.Flower structures that encourage cross-fertilization and reduce self-fertilization were studied widely. After considerable debate during this period, it was also acknowledged that some plants are adapted to regular self-fertilization and have floral syndromes that contrast strongly with those associated with outcrossing (see Darwin [ 1876] and Henslow [1879] for divergent viewpoints and Miuller's [1883] brilliant resolution of the issue).Floral biology became popular again in the second half of the twentieth century. In this period, genetic studies have dominated comparisons of self-and cross-fertilization. The genetic studies have provided much-needed empirical information on several aspects of self-and cross-fertilization, including frequencies of self-fertilization (Barrett and Eckert 1990), the expression and causes of inbreeding depression (Charlesworth and Charlesworth 1987; Barrett and Charlesworth 1991), and the genetic structures of outcrossing and selfing populations (Brown 1990; Hamrick and Godt 1990; Ritland 1990). The theoretical effects of the mating system on the genetic structures of regularly selfing and outcrossing populations have also been analyzed widely (Brown 1990;Ritland 1990).

Outcrossing rates of individual Mimulus ringens genets are correlated with anther–stigma separation

“…Increased anther-stigma separation may reduce the amount of self-pollen deposited on the stigma, potentially increasing the proportion of ovules fertilized by outcross pollen (Ennos, 1981;Webb & Lloyd, 1986;Lloyd & Schoen, 1992).…”

CORRELATED EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION: AN EXPERIMENTAL STUDY OF NINE POPULATIONS OFAMSINCKIA(BORAGINACEAE)

“…Third, ecological factors may cause the selting rate to vary among seasons, not only rendering anyone season unrepresentative, but also slowing attainment of equilibrium. Fourth, ecological factors (pollination assurance, pollen and seed discounting, changes in population size, mode of self-pollination [Lloyd and Schoen 1992;Lloyd 1992]) may overwhelm the role of inbreeding depression in the evolution of the selting rate. Finally, the dynamics of the coevolution may be driven by associations between loci causing inbreeding depression and those changing the level of selting in such a way that no correlation exists between them (see Uyenoyama et al 1993).…”

Biología floral y reproductiva de Escallonia pulverulenta (Ruiz et Pav.) Pers. (Escalloniaceae) y su relación con los visitantes florales

“…Valores cercanos a 1,0 indican que los polinizadores están presentes de forma adecuada, mientras que valores < 1,0 indican limitación de polen mediada por polinizadores (Bierzychudek 1981, Young & Young 1992. Por último, se calculó el índice de autofertilidad como la razón entre el número de semillas producidas por polinización automática y polinización cruzada manual (Lloyd & Schoen 1992). En los 3 casos se calculó un valor promedio de cada índice obtenido para cada planta (n=11) y se evaluó si difería significativamente de 0 ó 1 por test de Wald (Fox 2002).…”

“…De este modo, la polinización cruzada entre individuos genéticamente diferentes (i.e. xenogamia) incrementa las variaciones genéticas entre individuos y su potencial de adaptación al ambiente, aunque su costo es la pérdida de potencial reproductivo si los polinizadores son escasos (Lloyd & Schoen 1992, Eckert & Barrett 1994, Charlesworth 2002. Por otro lado, la autopolinización por polen de la misma flor (i.e.…”

“…Atributos de la biología floral, como el período de floración, el tiempo de receptividad del estigma, el despliegue floral, la maduración de las anteras y la longevidad floral, influyen en la transferencia de polen por los polinizadores, en las tasas de geitonogamia y en el tipo de polinizador asociado (Primack 1980, 1985, Zimmerman 1988, Lloyd & Schoen 1992, Harder & Barrett 1995, Barrett 2003. Por ejemplo, especies protándricas son frecuentemente polinizadas por abejas o moscas, mientras que las protoginias son polinizadas por escarabajos o por el viento (Sargent & Otto 2004).…”