What impact does the following conditions have on the sexual reproduction outcome in plants?
Flower do not open even after its reproductive structures are fully matured?
Answers
Answer:
Explanation:
Among higher plants, zygotes formed by sexual fertilization are always transmitted to the next generation via seeds. Also asexually derived embryogenic material may be transmitted by seeds between generations. Such asexual seed production, apomixis, is common in many plant families . However, unlike sexual reproduction, asexual reproduction may use many other kinds of plant structures to start new generations, e.g. bulbils, runners and rhizomes. Collectively, all specialized reproductive modules will here be called propagules. Many asexual propagules are morphologically and functionally very different from seeds, but some must be regarded as highly seed‐like, as for example the bulbils produced in the flower‐heads of many Liliaceae species.
Explanation:
The balance between sexual and asexual propagule production is studied in an evolutionary model where plants produce the two kinds of propagule in genetically determined proportions. The male function of plants producing asexual propagule can be varied, and the sexual and asexual propagule carry different probabilities to turn into new reproductive individuals. These fitness may vary over years. The evolution of the population’s reproductive system is studied assuming modifier alleles with small effects.
In this setting a balanced, mixed reproductive system can evolve, but only if the difference in fitness between the sexual and asexual propagule varies over years. When the two kinds of propagule are very similar to each other, as is often the case with sexual and asexual seed formation, evolution will tend towards a state dominated by the one or the other reproductive system.
Among higher plants, zygotes formed by sexual fertilization are always transmitted to the next generation via seeds. Also asexually derived embryogenic material may be transmitted by seeds between generations. Such asexual seed production, apomixis, is common in many plant families .However, unlike sexual reproduction, asexual reproduction may use many other kinds of plant structures to start new generations, e.g. bulbils, runners and rhizomes. Collectively, all specialized reproductive modules will here be called propagule . Many asexual propagule are morphologically and functionally very different from seeds, but some must be regarded as highly seed‐like, as for example the bulbils produced in the flower‐heads of many species.It appears as if sexual reproduction does not combine equally well with the different methods for asexual reproduction. While seed‐producing plants normally are either predominantly sexual or asexual, plants that reproduce asexually with other kinds of propagule may well combine this with extensive sexual seed set. The difference is well illustrated by Potential argentena, hoary cinque‐foil, wild garlic, two common European plants that often grow in close proximity.
Potential argentena occurs in two forms. The first is diploid and fully sexual (seling with some breeding); the second is hexaploid and apomictic. The two forms are next to indistinguishable in the field and are often found in sympatry. The apomictic form is pseudogamous (i.e. it requires pollen for seed formation) and its pollen production appears to be normal, judging from how easily such plants can fertilize diploids. On the female side, there seems to be no regular sexual process.
The difficulty with combining sexual and asexual seed set in Potentilla argentea can be contrasted with the situation in Allium vineale. In this species some, most or all the flowers in the flower‐heads are transformed into bulbils. The bulbils are asexually produced and disperse like large seeds. The other flower stalks in the heads develop into normal flowers. These flowers take part in sexual fertilizations (mediated by insects) with both male and female functions and they produce viable seeds. Though the frequency of bulbils per head varies between individuals and populations, producing fair numbers of both seeds and bulbils seems to be the norm for A. vineale plants (information from 16; 18; Ceplitis, unpublished work).
How is it possible for evolution to stably combine sexual and asexual reproduction with bulbils, while balanced combinations of sexual and asexual (apomictic) seed set seems rare if not absent?
This question has two aspects of which the first is genetic. Even though bulbils made from transformed flower structures may appear similar to seeds, there are some crucial differences between them. The most important is that by transforming a flower into a bulbil, not only is the resulting propagule different, but the male sexual function of the flower is concomitantly lost. This is not necessarily the case for flowers producing seeds apomictically, where the male function often remains intact. Thus, the genetic transmission systems are often radically different in seed and bulbil forming asexual plants. (The difference is easily envisaged by considering a rare sexual type in a population dominated by asexuals. Under apomixis, almost all pollen fertilizing the plant would come from apomictic plants; under bulbil formation, on the other hand, most pollen would come from other plants with both male and female sexual functions.)
Also sexually and apomictically produced seeds should differ in their probabilities of giving rise to viable individuals due to the intrinsic advantage (or disadvantage) of carrying recombined genetic material. This kind of effect is, however, unlikely to vary as much over years as the fitness differential between sexual seeds and asexual bulbils.
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