Plant Reproductive Systems
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Across the plant kingdom can be found a wide variety of reproductive systems. Here I will simply give the terminology and a short description of some of those systems. But first, some flower terminology. A stamen (or androecium) is the male component of a flower and is composed of the filament and the anther. The filament is a stalk which connects to the anther, and the anther contains the pollen. Pollen is a microgametophyte and it contains a generative and a vegetative nucleus. Pollen grains are derived from ‘pollen mother cells’ which undergo meiosis. A pistil (or carpel) is the female component of a flower and is composed of the stigma, style, and ovary. The stigma acts as a receptor for pollen (on which pollen germinates to form pollen tubes). The style is a stalk which connects the stigma to the ovary and through which pollen tubes grow. The ovary is located at the basal region of the pistil and it contains the ovules (which upon fertilization develop to form seeds). A flower is the reproductive unit of angiosperms. There are, generally, two types of flowers. An ‘imperfect’ flower has either male or female components, but not both (i.e. flowers can be unisexual -- either male or female). A ‘perfect’ flower has both male and female components. This is also known as a ‘hermaphrodite’ or ‘bisexual’ flower. The diagram below shows a simplified structure of a perfect flower.
Dellaporta and Calderon-Urrea (1993) have listed and defined a variety of terms used to describe the modes of sexuality at different levels in plants. I will reproduce that list (with some modifications) below: Individual Flowers:
Individual Plants:
Plant Populations
Hermaphroditism is very common in plants -- about 70% of flowering plants are hermaphroditic, while only about 5% are dioecious and 7% are monoecious. About 7% of species exhibit gynodioecy or androdioecy, while 10% contain both unisexual and bisexual flowers. It is thought that flowering plants evolved from a common hermaphrodite ancestor, and that dioecy evolved from hermaphroditism. I have gone into more detail, with respect to the evolution of dioecy, in my field report on Sagittaria latifolia.
Plant breeding systems have been viewed as mechanisms to promote outcrossing (or as mechanisms to prevent inbreeding). As mentioned above, hermaphrodites are common in plants, while much fewer species are dioecious. Dioecy is easily seen as a mechanism to preventing inbreeding -- that is, it prevents selfing, since male and female flowers exist on separate plants. Hermaphrodites, on the hand, must evolve other mechanisms to prevent selfing, or at least, to reduce inbreeding depression (i.e. the accumulation of deleterious, recessive alleles). Below is a list of some types of breeding systems (including ones I already mentioned above):
Since I have already outlined the various types of sex expression above (i.e. monoecy, dioecy, etc), I will now focus on the other two sections, spatial/temporal patterns and self-incompatibility. Dichogamy refers to temporal separation in male and female functions of bisexual (or perfect flowers). Protandry occurs when pollen sheds first, before the stigmas become receptive. Protogyny, on the other hand, occurs when the stigmas become receptive first, before pollen is released. Protandry is more common than protogyny. In order for dichogamy to be an effective outcrossing mechanism, all flowers on the plant must be synchronous (i.e. if protandry occurs, all flowers must release their pollen before the stigmas become receptive). Otherwise, pollen can be transfered from one flower to another on the same plant (this is referred to as “geitonogamy”). Dichogamy can also work if there is a sequential developmental pattern of male and female flowers (i.e. certain pollinators tend to move ‘up’ a plant -- if flowers are male first, and develop into female flowers starting from the bottom of the plant, then the chances of geitonogamy can be reduced). Herkogamy is the spatial separation of styles and anthers in a perfect flower (Fig-1). This system is apparently widespread, however, geitonogamous pollen transfer is possible. In flowers that do not have spatial separation of styles and anthers, self pollination (referred to as “autogamous” pollination) can occur.
Sex-switching simply refers to a plant that is either male or female at one time, and then later (perhaps later in the growing season), the plant switches to the opposite sex. Self-incompatibility (SI) is the inability of a hermaphroditic plant (that is capable of producing functional gametes) to set seeds when it is self-fertilized or fertilized by ‘like’-individuals. Self-compatibility (SC), on the other hand, refers to a plant is is capable of setting seeds when self-fertilized. There are two types of self-incompatibilty: homomorphic SI and heteromorphic SI. Homomorphic SI occurs when there is a self-incompatible system in a species where all individuals in a population look identical (that is, all flowers are morphologically similar). Heteromorphic SI (or heterostylous SI) occurs when there are two or morph different morphs in the population (i.e. in perfect flowers, the styles and stamens are different lengths). Homomorphic SI can be one of two types: 1) gametophytic, or 2) sporophytic. In the gametophytic type, there is a single locus (the ‘S’-locus) which has many different alleles (e.g. S1, S2, S3, S4, etc). If a pollen grain has an allele that is also possessed by the recipient, the pollen is rejected (i.e. fertilization does not occur) (Fig-2). Similarly, pollen is rejected in sporophytic SI if the recipient has either of the alleles present in the pollen donor (Fig-3).
It is called ‘gametophytic’ SI, because only one parental allele is expressed in the pollen. Conversely, both parental alleles are expressed in the pollen grain in sporophytic SI (even though the pollen may only contain one allele -- thus, there is a parental effect with the sporophytic system). Gametophytic SI is found in Oenothera sp., as well as in the Solanaceae (potato family), Papaveraceae (poppy family), Poaceae (grass family), Ranunculaceae (buttercup family), and the Rosaceae (rose family, e.g. apples). Sporophytic SI is found in the Brassicaceae (mustard family), Asteraceae (aster family), and the Convolvulaceae. It had been oringially thought that self-incompatibility systems were responsible for the success of the angiosperms (i.e. SI was considered to be an ancestral trait in the angiosperms). By comparing these different systems, it is now known that SI arose independently on numerous occassions (e.g. some genes involved in these systems have been sequenced and compared -- these genes encode very different products in different species). Heteromorphic (or heterostylous) SI is the occurence of different reproductive morphs in a population. Distyly is fairly common (i.e. it has been found in 25 different families, such as the Primulaceae, Polygonaceae, Menyanthaceae, and Turneraceae), whereas tristyly is very rare (i.e. it has been demonstrated in only three families: the Oxalidaceae, Lythraceae, and Pontederiaceae, and possibly in the Amaryllidaceae). There are a number of differences in the different pollen and style types, which I will not discuss here. In Distyly, there are only two morphs (longs and shorts) (Fig-4). Tristyly, on the other hand, has three morphs (longs, mids, and shorts) (Fig-5). The arrows/lines indicate which crosses result in fertilization
One question concerns the evolution of these heteromorphic SI systems: if the sole function of these systems is to promote outcrossing, then why are there both the presence of different morphs and of the self-incompatibility systems? The SI system alone should be enough to promote outcrossing, so why is there a floral polymorphism? One explanation (which apparently comes from Darwin), is that the polymorphism is not a mechanism for promoting outcrossing, but for increasing fecundity (e.g. seed set or pollination efficiency: pollen from one morph sticks to a certain body part on the pollinator, and this is then transferred to a stigma that is at the same relative position). This idea has been confirmed in recent years: distyly and tristyly enhance the ability of getting the ‘right’ pollen onto the ‘right’ stigmas. In summary, there are a wide variety of reproductive systems in plants. For evolutionary studies, the trick is to identify which systems are present in a particular group of species, and then compare them to see which ones have similar or identical mechanisms. This can then be used to infer or confirm phylogenetic relationships between species, and/or to determine which systems evolved once or more than once.
References:
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