The Evolution of the Genome by Unknown

Author:Unknown
Language: eng
Format: epub
Publisher: Elsevier Science
Published: 2005-08-15T00:00:00+00:00

POLYPLOID FORMATION AND ESTABLISHMENT

MECHANISMS AND CHANCES OF FORMATION

Although enormous strides have been made in the study of some aspects of the genetics and evolution of polyploids, their exact mechanisms of origin remain poorly understood. Polyploidy often is described as “chromosome doubling,” which implies a somatic (nonreproductive) event during formation. A somatic chromosome doubling event could occur in a zygote or in developing seedlings or even in active apical meristematic tissues. Both zygotic and meristematic chromosome doubling would immediately result in polyploidy. Importantly, however, it has been noted that critical evidence in support of chromosome doubling in the zygote is lacking (de Wet, 1980), and spontaneous somatic (meristematic) chromosome doubling is considered very rare (e.g., Nasrallah et al., 2000; Grant, 2002). Thus, although “chromosome doubling” in a strict somatic sense may be one method of polyploid formation, it is not considered the most common mechanism. Instead, Harlan and de Wet (1975) and de Wet (1980) proposed that a more common method of polyploidization is via the formation and fusion of unreduced gametes.

The production of pollen and egg cells is a complex process, and the failure of chromosome reduction during meiosis has been observed in many plant species. That is, as a result of several possible meiotic mishaps, a pollen or egg cell can be produced that is not haploid, but rather has the same “unreduced” chromosome number as the parent cell. As reviewed by several authors (de Wet, 1980; Bretagnolle and Thompson, 1995; Ramsey and Schemske, 1998), unreduced gametes are probably produced by most individuals. Obviously, the union of two such unreduced gametes would result in an instantaneous polyploid. However, the rate of production of unreduced gametes is quite low (at most, just a few percent of all gametes produced), such that the probability of an unreduced egg being fertilized by an unreduced sperm (a process termed “bilateral polyploidization”) (Stebbins, 1971) is relatively small. It is therefore more likely that the formation of higher polyploids proceeds via the intermediate of triploidy (Bretagnolle and Thompson, 1995; Ramsey and Schemske, 1998). Thus the first step would be fertilization involving haploid pollen and an unreduced (diploid) egg (or, less frequently, vice versa) to yield a triploid zygote (“unilateral polyploidization”). If this triploid is viable and fertile—that is, it survives to maturity and produces functional reproductive structures—it may generate a number of triploid eggs. If these, in turn, back-cross to a normal diploid plant (and hence are fertilized by a haploid pollen grain), the result would be a tetraploid. This triploid step toward tetraploid formation is sometimes referred to as a “triploid bridge,” and there is increasing evidence that this is an important stage in polyploid formation (Bretagnolle and Thompson, 1995; Ramsey and Schemske, 1998; Husband, 2004).

Both de Wet (1980) and Ramsey and Schemske (1998) pointed out that adverse growing conditions may increase the frequency with which unreduced gametes are produced (e.g., Clausen et al., 1940; Grant, 1952). The genotype of the individual also seems to play a role, with some individuals and populations more prone to generating unreduced gametes than others.

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