Heterostyly is a unique form of polymorphism and herkogamy in flowers. In a heterostylous species, two or three morphological types of flowers, termed "morphs", exist in the population. On each individual plant, all flowers share the same morph. The flower morphs differ in the lengths of the pistil and stamens, and these traits are not continuous. The morph phenotype is genetically linked to genes responsible for a unique system of self-incompatibility, termed heteromorphic self-incompatibility, that is, the pollen from a flower on one morph cannot fertilize another flower of the same morph.
Heterostylous plants having two flower morphs are termed "distylous". In one morph (termed "pin", "longistylous", or "long-styled" flower) the stamens are short and the pistils are long; in the second morph (termed "thrum", "brevistylous", or "short-styled" flower) the stamens are long and the pistils are short; the length of the pistil in one morph equals the length of the stamens in the second morph, and vice versa. [1] [2] Examples of distylous plants are the primrose and many other Primula species, [1] [2] buckwheat, flax and other Linum species, some Lythrum species, [3] and many species of Cryptantha . [4]
Heterostylous plants having three flower morphs are termed "tristylous". Each morph has two types of stamens. In one morph, the pistil is short, and the stamens are long and intermediate; in the second morph, the pistil is intermediate, and the stamens are short and long; in the third morph, the pistil is long, and the stamens are short and intermediate. Oxalis pes-caprae , purple loosestrife ( Lythrum salicaria ) and some other species of Lythrum are trimorphic. [3]
The lengths of stamens and pistils in heterostylous flowers are adapted for pollination by different pollinators, or different body parts of the same pollinator. Thus, pollen originating in a long stamen will reach primarily long rather than short pistils, and vice versa. [1] [2] When pollen is transferred between two flowers of the same morph, no fertilization will take place, because of the self-incompatibility mechanism, unless such mechanism is broken by environmental factors such as flower age or temperature. [5]
Heterostyly has evolved independently in over 25 different plant families, including the Oxalidaceae, Primulaceae, Pontederiaceae, and the Boraginaceae. [6] [7] These families do not exhibit heterostyly across all species, and some families can exhibit both mating systems, such as among species in the genus Eichhornia (Pontederiaceae). For example, Eichhornia azurea exhibits distyly, whereas another species in the same genus, Eichhornia crassipes, is tristylous. [8]
Heterostyly is thought to have evolved primarily as a mechanism to promote outcrossing. Several hypotheses have been proposed to explain the repeated independent evolution of heterostyly as opposed to homostylous self-incompatibility: 1) that heterostyly has evolved as a mechanism to reduce male gamete wastage on incompatible stigmas and to increase fitness through male function through reciprocal herkogamy; 2) heterostyly evolved as a consequence of selection for heteromorphic self-incompatibility between floral morphs in distylous and tristylous species; and, 3) that the presence of heterostyly in plants reduces the conflict that might occur between the pollen dispersal and pollen receipt functions of the flower in a homomorphic animal-pollinated species. [9]
Heterostyly is most often seen in actinomorphic flowers presumably because zygomorphic flowers are effective in cross- pollination. [9]
Models
Current models for evolution include the pollen transfer model and the selfing avoidance model.
The pollen transfer model proposed by Lloyd and Webb in 1992 is based on the efficacy of cross-pollen transfer, and suggests that the physical trait of reciprocal herkogamy evolved first, and then the diallelic incompatibility arose afterwards as a response to the evolution of the reciprocal herkogamy. [6] This model is similar to Darwin's 1877 idea that reciprocal herkogamy evolved as a direct response to the selective forces that increase accuracy of pollen transfer. [10]
The alternative model - the selfing avoidance model - was introduced by Charlesworth and Charlesworth in 1979 using a population genetic approach. The selfing avoidance model assumes that the self-incompatibility system was the first trait to evolve and that the physical attribute of reciprocal herkogamy evolved as a response to the former. [11]
Genetic determination
The supergene model describes how the distinctive floral traits present in distylous flowers can be inherited. This model was first introduced by Ernst in 1955 and was further elaborated by Charlesworth and Charlesworth in 1979. Lewis and Jones in 1992 demonstrated that the supergene consists of three linked diallelic loci. [11] [12] [13] The G locus is responsible for determining the characteristic of the gynoecium which includes the style length and incompatibility responses, the P locus determines the pollen size and the pollen's incompatibility responses, and finally the A locus determines the anther height. These three diallelic loci compose the S allele and the s alleles segregating at the supergene S locus, which is notated as GPA and gpa, respectively. There have been other propositions that there are possibly 9 loci responsible for the distyly supergene in Primula, but there has been no convincing genetic data to support this.
Additionally, supergene control is implied for tristyly, but there is no genetic evidence available to support it. A supergene model for tristyly would require the occurrence of two supergenes at the S and M loci. [9]
Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, and bats; water; wind; and even plants themselves, when self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species, it can produce hybrid offspring in nature and in plant breeding work.
Lythraceae is a family of flowering plants, including 32 genera, with about 620 species of herbs, shrubs, and trees. The larger genera include Cuphea, Lagerstroemia (56), Nesaea (50), Rotala (45), and Lythrum (35). It also includes the pomegranate and the water caltrop. Lythraceae has a worldwide distribution, with most species in the tropics, but ranging into temperate climate regions as well.
Self-pollination is a form of pollination in which pollen from the same plant arrives at the stigma of a flower or at the ovule. There are two types of self-pollination: in autogamy, pollen is transferred to the stigma of the same flower; in geitonogamy, pollen is transferred from the anther of one flower to the stigma of another flower on the same flowering plant, or from microsporangium to ovule within a single (monoecious) gymnosperm. Some plants have mechanisms that ensure autogamy, such as flowers that do not open (cleistogamy), or stamens that move to come into contact with the stigma. The term selfing that is often used as a synonym, is not limited to self-pollination, but also applies to other type of self-fertilization.
Plant reproductive morphology is the study of the physical form and structure of those parts of plants directly or indirectly concerned with sexual reproduction.
Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals (dioecy), and their various modes of spatial (herkogamy) and temporally (dichogamy) separation.
Lythrum is a genus of 38 species of flowering plants native to the temperate world. Commonly known as loosestrife, they are among 32 genera of the family Lythraceae.
Mitchella repens is the best known plant in the genus Mitchella. It is a creeping prostrate herbaceous woody shrub occurring in North America belonging to the madder family (Rubiaceae).
A supergene is a chromosomal region encompassing multiple neighboring genes that are inherited together because of close genetic linkage, i.e. much less recombination than would normally be expected. This mode of inheritance can be due to genomic rearrangements between supergene variants.
Sequential hermaphroditism is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. In particular, a sequential hermaphrodite produces eggs and sperm at different stages in life. Sequential hermaphroditism occurs in many fish, gastropods, and plants. Species that can undergo these changes do so as a normal event within their reproductive cycle, usually cued by either social structure or the achievement of a certain age or size. In some species of fish, sequential hermaphroditism is much more common than simultaneous hermaphroditism.
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. Flowers produce gametophytes, which in flowering plants consist of a few haploid cells which produce gametes. The "male" gametophyte, which produces non-motile sperm, is enclosed within pollen grains; the "female" gametophyte is contained within the ovule. When pollen from the anther of a flower is deposited on the stigma, this is called pollination. Some flowers may self-pollinate, producing seed using pollen from the same flower or a different flower of the same plant, but others have mechanisms to prevent self-pollination and rely on cross-pollination, when pollen is transferred from the anther of one flower to the stigma of another flower on a different individual of the same species.
Tristyly is a rare floral polymorphism that consists of three floral morphs that differ in regard to the length of the stamens and style within the flower. This type of floral mechanism is thought to encourage outcross pollen transfer and is usually associated with heteromorphic self-incompatibility to reduce inbreeding. It is an example of heterostyly and reciprocal herkogamy, like distyly, which is the more common form of heterostyly. Darwin first described tristylous species in 1877 in terms of the incompatibility of these three morphs.
Xenogamy is the transfer of pollen grains from the anther to the stigma of a different plant. This is the only type of cross pollination which during pollination brings genetically different types of pollen grains to the stigma.
Papilionaceous flowers are flowers with the characteristic irregular and butterfly-like corolla found in many, though not all, plants of the species-rich Faboideae subfamily of legumes. Tournefort suggested that the term Flores papilionacei originated with Valerius Cordus, who applied it to the flowers of the bean.
Arcytophyllum is a genus of flowering plants in the family Rubiaceae. The genus contains 18 species, distributed from New Mexico to Bolivia.
Turnera subulata is a species of flowering plant in the passionflower family known by the common names white buttercup, sulphur alder, politician's flower, dark-eyed turnera, and white alder. Despite its names, it is not related to the buttercups or the alders. It is native to Central and South America, from Panama south to Brazil. It is well known in many other places as an introduced species, such as Malaysia, Indonesia, several other Pacific Islands, the Caribbean, and Florida in the United States.
Hypericum aegypticum is a species of flowering plant of the St. John's wort family (Hypericaceae) which is native to the Eastern Mediterranean. It was described by Carl Linnaeus in the second volume of his Species Plantarum in 1753, who named it after Egypt despite it not being distributed there. The plant is commonly known as shrubby St. John's wort or Egyptian St. John's wort in English. Like other members of section Adenotrias, it is found among limestone rocks in coastal areas. While it has been evaluated as threatened on the island of Malta, the species has no legal protections.
The monocots are one of the two major groups of flowering plants, the other being the dicots. In order to reproduce they utilize various strategies such as employing forms of asexual reproduction, restricting which individuals they are sexually compatible with, or influencing how they are pollinated. Nearly all reproductive strategies that evolved in the dicots have independently evolved in monocots as well. Despite these similarities and their close relatedness, monocots and dicots have distinct traits in their reproductive biologies.
Distyly is a type of heterostyly in which a plant demonstrates reciprocal herkogamy. This breeding system is characterized by two separate flower morphs, where individual plants produce flowers that either have long styles and short stamens, or that have short styles and long stamens. However, distyly can refer to any plant that shows some degree of self-incompatibility and has two morphs if at least one of the following characteristics is true; there is a difference in style length, filament length, pollen size or shape, or the surface of the stigma. Specifically these plants exhibit intra-morph self-incompatibility, flowers of the same style morph are incompatible. Distylous species that do not exhibit true self-incompatibility generally show a bias towards inter-morph crosses - meaning they exhibit higher success rates when reproducing with an individual of the opposite morph.
Cryptic self-incompatibility (CSI) is the botanical expression that's used to describe a weakened self-incompatibility (SI) system. CSI is one expression of a mixed mating system in flowering plants. Both SI and CSI are traits that increase the frequency of fertilization of ovules by outcross pollen, as opposed to self-pollen.
Oxalis alpina is a herbaceous perennial plant also known by its common name alpine woodsorrel. It is a species belonging to the genus Oxalis.O. alpina is found in North America and Central America from Guatemala to the southwestern United States.
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