Hétérostylie

The hétérostylie is a floral polymorphism under control Génétique appeared independently in 24 families of plants with flowers such as for example, the Primevère. Each plant produces only one morphe floral. The morphes differ by the length from the cheesecloth S and from the Style S. Moreover, they often differ by the size and the number of grains of produced Pollen and sometimes by the structure of the Exine, the color of pollen, the presence or not of Amidon in pollen, the structure of the stigmatic papillae and the size of the Corolle (Ganders, 1979; Dulberger, 1992). The distylic plants have two morphes and the tristylic plants have three of them. These last are rarer; one finds them only at the Lythraceae, Oxalidaceae and Pontederiaceae and two other families in whom them presence was not confirmed yet: the Connaraceae and Linaceae (Ganders, 1979).

In the distylic species, the flowers (and thus, by abuse language, plants) are is of brévistyle type (“English thrum”) with styles shorter than cheesecloths (tabl. 1 and fig. 1), is of longistyle type (“English pine”) with styles longer than cheesecloths.

In the tristylic species, the flowers of each morphe are characterized by three different levels of which two are occupied by anthères and the third, by a mark (fig. 1). The flowers longistyles thus have a long style and average and short cheesecloths. The flowers mésostyles have a style average length and cheesecloths the long and short flowers brévistyles have a short style and cheesecloths average and long length.

In the majority of the cases, different the morphes from the species hétérostyles is compatible between them (one speaks about legitimate crossings), whereas pollination between of the same plants morphe and car-pollination are incompatible (one speaks about illegitimate crossings) (Richards, 1997) (cfr. I.2.). For example, in the case of the distylic species, the pollen produced by long cheesecloths is compatible with the short styles and conversely. The population is thus divided into two compatible groups.

Adaptive significance

Already Darwin proposed in 1877 that the reciprocal positioning of the Anthère S and mark S of the floral morphes of the plants hétérostyles would make it possible to support the Allopollinisation while placing the pollen of a morphe on part of the body of the pollinating insect corresponding precisely to the zone which will come into contact with the marks of another morphe. It observed besides that when it introduced various objects like hairs, needles or Proboscis of Abeilles dead in flowers of Primula spp., the grains of pollen were deposited on different zones according to the morphe. Other authors, thereafter, examined the distribution of pollen brévistyle and visiting longistyle on insects of the distylic plants. The pollen of both morphes was on parts different from the body of pollinating visiting Fagopyrum esculentum (Rozov & Skrebtsova, 1958, cité by Namai, 1990), Pulmonaria (Olesen, 1979, cité by Lloyd & Webb, 1992b) and Cratoxylum (Lewis, 1982, cité by Lloyd & Webb, 1992b). At Eichhornia paniculata (Pontederiaceae), a tristylic species, the tubular flowers are car-compatible and yet 77% of fecundations are inter-morphes (Kohn & Barrett, 1992).

The car-incompatibility almost always associated with the hétérostylie (cfr. I.2.1.3.C.) makes unfruitful any standard pollination between morphe in the same way. Baker (1964) recognizes that the pollen deposited on an incompatible mark is wasted because it does not take part in the fitness of the plant. It must thus exist a pressure of selection tending to decrease the incompatible transfers and to increase the compatible transfers.

One observes at the majority of the distylic species that the marks of the flowers longistyles collect more pollen than those of the flowers brévistyles (Dulberger, 1992). That is allotted to greatest accessibility, with the contact with insects, long styles that short styles (Ganders, 1975). In the majority of the species hétérostyles, short cheesecloths of the flowers longistyles produce grains of more and smaller pollens (Dulberger, 1992). According to Ganders (1979), the more important production in grains of pollen of the flowers longistyles would be used to compensate for the weak deposit of pollen on the marks of the flowers brévistyles. Those receive, consequently, a greater proportion of compatible pollen. The lower size of these grains would be only one physiological means to produce a more significant number of it (Ganders, 1975).

Control genetic

The loci controlling the reactions of incompatibilities of the distylic species are closely related to the loci controlling heteromorphism, so that they are inherited together like only one gene behaving the made-to-order of a simple factor mendélien, called supergene S (Charlesworth & Charlesworth, 1979; Ganders, 1979). This one orders differentiation in plants brévistyles, Hétérozygote S (S), and longistyles, recessive Homozygote S (S) (Ganders, 1979). That implies that a 1:1 relationship between the two morphes is maintained from generation to generation. Being given that the crossings between plants brévistyles are not compatible, it cannot exist plants brévistyles homozygotes S. This genotype naturally appears however in the populations of the rare distylic species autogames like Amsinckia spectabilis (Boraginaceae) (Ganders, 1975).

It is Dowrick (1956) which advanced the first this concept of supergene while working on Primula obconia. She proposed three under-genes then: G, P and have respectively controlling the length of the style, the size of pollen and the length of the net. By irradiating Akène S of buckwheat, Sharma & Boyes (1961) confirmed that this one also followed the model of supergene. They supplemented the first three under-genes by two others controlling the reaction of incompatibility of the style (Is) and pollen (IP). The supergene S would differentiate both thus Génotype S.

One could speak about a system multiallelic of autoincompatibility but the loci would be so close one to the other on the chromosome which one should regard the system as diallelic. The recombination S between under-genes are rare.

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