The Ecological and Evolutionary Consequences of Clonality for Plant Mating

Abstract

Many flowering plants exhibit dual reproductive modes, producing both sexual and asexual offspring. The commonest form of asexual reproduction is clonal growth, in which vegetative modules (ramets) are produced by the parental genotype (genet). In plants, sexual and asexual reproduction usually occur simultaneously, and this can lead to allocation trade-offs and antagonism between reproductive modes. Our review considers the ecological and evolutionary consequences of functional interactions between clonal reproduction and pollination and mating. Clonal reproduction is commonly associated with mass flowering, restricted pollen dispersal, and geitonogamous self-pollination, processes that can result in inbreeding depression and pollen discounting. We review evidence for the correlated evolution of clonality and sexual systems, particularly self-incompatibility, and identify several floral mechanisms that function to reduce mating costs by limiting selfing and pollen discounting. We conclude by discussing the loss of sexuality in clonal plants and consider the genetic and environmental basis of sexual dysfunction. 193 Review in Advance first posted online on August 10, 2010. (Changes may still occur before final publication online and in print.) Changes may still occur before final publication online and in print.

Influences of clonality on plant sexual reproduction

“…However, clonality is widely distributed among angiosperms, and large clone size has net benefits to fitness because of increased resource capture, greater competitive ability, and higher reproductive success through both female and male function. An important unresolved issue concerns the quality of mating that arises from different clonal strategies and the extent to which increases in clone size inevitably result in greater levels of geitonogamous selfing and reduced fitness (5,7,8). A recent study casts doubt on this frequent assumption by examining alternative strategies for the deployment of flowers and the influence that flower distribution has on pollinator behavior and pollen dispersal (70).…”

“…Three particular influences have been identified (5). First, if there are strong trade-offs between investment in sexual and vegetative reproduction, rapid clonal expansion may limit allocation to flowering and seed production (6).…”

“…Second, vegetative reproduction has the potential to interfere with pollination and mating, resulting in reductions in the quantity and quality of offspring. This situation can occur when clonal dispersal disrupts the functioning of sexual systems, leading to the absence of mating groups required for outcrossing, or where very large clones experience fitness costs (inbreeding depression and pollen discounting) associated with geitonogamous (between flower) self-pollination because of large floral displays (5,7,8). Finally, in highly clonal populations in which sexual reproduction is very limited or absent, there is growing evidence that the accumulation of somatic mutations may result in reductions in fertility, potentially leading to the loss of sex.…”

Consequences of clonality for sexual fitness: Clonal expansion enhances fitness under spatially restricted dispersal

“…In particular, it has been argued that the production of numerous partially or fully autonomous clonal modules (i.e., ramets) should interfere with mating success because there is a nonzero chance that sperm (or pollen) will encounter the receptive tissues associated with the female function of the same genetic individual (i.e., the genet). All else being equal, the larger the clone, the greater the chance that this kind of mating interference should occur (4,5). The negative effects of such intraclonal mating is expected to occur through reductions in the number of offspring sired on other genets (i.e., outcross siring success, corresponding with the fitness of individuals via their male function) and, in self-compatible organisms, through inbreeding depression (i.e., a reduction in offspring fitness, with negative effects on the fitness of individuals through their female function; e.g., ref.…”

“…The negative effects of such intraclonal mating is expected to occur through reductions in the number of offspring sired on other genets (i.e., outcross siring success, corresponding with the fitness of individuals via their male function) and, in self-compatible organisms, through inbreeding depression (i.e., a reduction in offspring fitness, with negative effects on the fitness of individuals through their female function; e.g., ref. 5). However, these forms of mating interference are simply by-products of attaining a larger size and are not exclusively associated with clonality (6).…”