Nonrandom mating (Botany)
Nonrandom mating in botany refers to specific mating behaviors among plants that deviate from the random pairing predicted by the Hardy-Weinberg theorem. This phenomenon can occur in three primary forms: positive assortative mating, negative assortative mating, and inbreeding. Positive assortative mating occurs when individuals share similar phenotypic traits, whereas negative assortative mating involves dissimilar individuals pairing. Inbreeding, a more common occurrence, happens when individuals mate with close relatives, increasing the likelihood of homozygosity in offspring.
Inbreeding can result in significant evolutionary consequences, such as inbreeding depression, which decreases overall fitness due to the accumulation of deleterious alleles. However, self-fertilization—an extreme form of inbreeding—can provide reproductive assurance for plants in isolated or rare circumstances, particularly among certain species that dominate disturbed environments. Conversely, assortative mating generally impacts specific reproductive traits, with negative assortative mating promoting diversity within the population. Understanding these mating patterns is essential in botany, as they influence genetic diversity, plant evolution, and the adaptive strategies of various species.
Nonrandom mating (Botany)
Categories: Genetics; reproduction and life cycles
There are three main ways for nonrandom mating to occur. First, positive assortative mating results when individuals and their mates share one or more phenotypic characteristics with themselves. Negative assortative mating occurs when individuals and their mates are dissimilar phenotypically. Inbreeding is a third form of nonrandom mating that occurs when individuals mate with relatives more often than would be expected by chance. For both inbreeding and assortative mating, genes combine in such a way that offspring genotypes differ from those that are predicted by the most basic population genetic model, described by the Hardy-Weinberg theorem. One assumption of the Hardy-Weinberg theorem, which predicts unchanging equilibrium values for genotype and allele frequencies, is that individuals mate at random. Unlike other violations of this model (such as natural selection, genetic drift, mutation), nonrandom mating affects genotype but not allele frequencies.
Inbreeding
Inbreeding is very common in many plant species for two main reasons. First, seed dispersal tends to follow a leptokurtic distribution, such that most seed falls near the parent plant. This results in near neighbors that are closely related and increases the probability that short-distance pollen movement will result in mating among relatives. In small populations with a limited number of potential mates, such matings between relatives are also common. Second, most flowering plants are hermaphroditic or monoecious. Thus, individual plants produce both male and female gametes and are capable of self-fertilization, the most extreme form of inbreeding.
The degree to which inbreeding occurs in a population depends upon the probability that an individual will mate with a relative or with itself. A plant’s mating system is characterized by the degree to which self-fertilization occurs and can range from complete outcrossing to complete self-fertilization, or selfing. While certain plant families tend to be characterized by a particular mating system (such as the inability to self-fertilize in the passionflower family, Passifloraceae), others exhibit great diversity in the levels of inbreeding among species (the grasses, Poaceae, and the legumes, Fabaceae).
For species with a mixed mating system, and which therefore engage in both selfing and outcrossing, the degree to which individual offspring are inbred is highly variable. Flowers with multiple ovules within an ovary can produce fruits with both selfed and outcrossed seeds. Some plants, such as violets (Viola) and jewelweed (Impatiens), produce morphologically distinct flowers for selfing and outcrossing.
Consequences of Inbreeding
Inbreeding has a larger evolutionary impact than assortative mating because it can affect all genes in the population. It can have negative consequences for plant survival and reproduction (fitness) because it tends to increase homozygosity and decrease heterozygosity. In response to these negative effects, collectively known as inbreeding depression, plants have evolved numerous adaptations to reduce levels of inbreeding. Although evidence for inbreeding depression in plants has been found, many species of plants are almost completely self-fertilizing and do not appear to suffer fitness consequences.
Under certain conditions, inbreeding may be advantageous. For example, rare plants or plants with rare pollinators may have few opportunities for outcrossing, and thus, self-fertilization provides a level of reproductive assurance. Many weedy plant species that tend to colonize disturbed sites are, in fact, capable of self-fertilization. Common crop weeds such as velvetleaf (Abutilon theophrasti) and shepherd’s purse (Capsella bursa-pastoris) predominantly self-fertilize. In these species, inbreeding may provide benefits that outweigh any associated costs. It has also been suggested that inbreeding species are better able to adapt to local environmental conditions because fewer maladapted genes from other populations would enter through outcrossing.
Reducing Inbreeding Depression
Inbreeding depression (which occurs when alleles that decrease fitness drift to fixation, causing a decrease in average fitness within a population) is reduced when plants are genetically or morphologically unable to self-fertilize. Genetic self-incompatibility, which is thought to occur in more than forty different plant families (for example, Brassicaceae, Solanaceae, and Asclepiadaceae) prevents mating between individuals that share certain genes that are involved in the interaction between pollen grains and the stigma or style. Morphological adaptations that reduce self-fertilization include those that separate anther and stigma maturation in time (protandry and protogyny) or space (heterostyly).
The individual hermaphroditic flowers of protandrous plants, such as phlox, shed their pollen prior to the time when the stigma on the same flower is receptive. Protogyny, which is less common than protandry, occurs when stigma receptivity occurs first (as in Plantago lanceolata). Dioecious plant species, such as date palms (Phoenix) and marijuana (Cannabis sativa), avoid selfing by having unisexual male and female flowers on separate individuals. In some hermaphroditic species, selfing is avoided when flower morphology favors crosses between certain flower phenotypes, as in the case of heterostyly.
Assortative Mating
Assortative mating generally affects only those traits important for reproduction. Many primrose (Primula) species are distylous, having two types of flowers. Flowers with the pin morphology have a tall style and relatively short stamens, while flowers with thrum morphology have a short style and long stamens. Insect-mediated pollen transfer results in matings between pin and thrum but not between two pin or two thrum plants. The result is nonrandom negative assortative mating. Unlike inbreeding, negative assortative mating tends to increase the level of heterozygosity in a population, at least for those traits that are involved in mate choice (such as relative style length).
Positive assortative mating, like inbreeding, results in increased homozygosity and decreased heterozygosity. Positive assortative mating for flowering time, for example, is common in many plant populations because individuals that flower early in the season will tend to mate with other early-flowering individuals.
Bibliography
Attenborough, D. The Private Life of Plants. Princeton, N.J.: Princeton University Press, 1995. Describes self-fertilization and plant adaptations that promote outcrossing. Includes color photographs.
Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman/Worth, 1999. This textbook describes effects of inbreeding and mechanisms plants employ to promote outbreeding.
Willson, M. F. Plant Reproductive Ecology. New York: John Wiley and Sons, 1983. Coverage of the evolution of sex and mating systems in plants. Topics include inbreeding, heterostyly, dioecy, and self-incompatibility.