Influences of clonality on plant sexual reproduction

Abstract

Flowering plants possess an unrivaled diversity of mechanisms for achieving sexual and asexual reproduction, often simultaneously. The commonest type of asexual reproduction is clonal growth (vegetative propagation) in which parental genotypes (genets) produce vegetative modules (ramets) that are capable of independent growth, reproduction, and often dispersal. Clonal growth leads to an expansion in the size of genets and increased fitness because large floral displays increase fertility and opportunities for outcrossing. Moreover, the clonal dispersal of vegetative propagules can assist "mate finding," particularly in aquatic plants. However, there are ecological circumstances in which functional antagonism between sexual and asexual reproductive modes can negatively affect the fitness of clonal plants. Populations of heterostylous and dioecious species have a small number of mating groups (two or three), which should occur at equal frequency in equilibrium populations. Extensive clonal growth and vegetative dispersal can disrupt the functioning of these sexual polymorphisms, resulting in biased morph ratios and populations with a single mating group, with consequences for fertility and mating. In populations in which clonal propagation predominates, mutations reducing fertility may lead to sexual dysfunction and even the loss of sex. Recent evidence suggests that somatic mutations can play a significant role in influencing fitness in clonal plants and may also help explain the occurrence of genetic diversity in sterile clonal populations. Highly polymorphic genetic markers offer outstanding opportunities for gaining novel insights into functional interactions between sexual and clonal reproduction in flowering plants.

Keywords: clonal growth; dioecy; geitonogamy; heterostyly; somatic mutations.

Fig. 1.

The influence of organs of clonal growth on the intermingling of genets and opportunities for outcrossing in clonal plants. The order from left to right reflects the distance that daughter ramets are usually produced from the parent plant; phalanx and guerrilla strategies represent two contrasting clonal strategies distinguished by this distance. Reprinted with permission from ref. .

Fig. 2.

Contrast in population style-morph structure between (A) E. azurea and (B) E. crassipes, two closely related, tristylous aquatic plants with different methods of clonal propagation and dispersal. Population surveys of E. azurea (26) and E. crassipes (27) were conducted in the Pantanal wetlands of Brazil and various regions of lowland tropical South America, respectively.

Fig. 3.

Contrasting patterns of variation in sex ratio in populations of different size for nonclonal (A and C) and clonal (B and D) dioecious species. A and B are the predicted patterns based on a metapopulation model with two founding individuals per population, and C and D are the observed patterns from populations of 9 and 14 nonclonal and clonal species, respectively (total populations n = 348; mean per species, nonclonal = 14.6; clonal = 15.5). Modified with permission from ref. .

Fig. 4.

The decline in male fertility with increasing clone age in the long-lived clonal tree P. tremuloides. For details of the estimation of relative male fertility and clone age, see ref. . Modified with permission from ref. .