Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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15 Reproductive Systems and Evolution in Vascular Plants
15 Reproductive Systems and Evolution in Vascular Plants
Pages 271-288
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From page 271... ...
Selfers and asexuals have fewer genotypes within populations than outcrossers with similar allele frequencies, and more genetic diversity in selfers and asexuals is a result of differences among populations than in sexual outcrossers. As a result of reduced levels of diversity, selfers and asexuals may be less able to respond adaptively to changing environments, and because genotypes are not mixed across family lineages, their populations may accumulate deleterious mutations more rapidly.
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From page 272... ...
Both the cost of outcrossing and reproductive assurance lead to an over-representation of selfers and asexuals in newly formed progeny, and unless sexual outcrossers are more likely to survive and reproduce, they eventually will be displaced from populations in which a selfing or asexual variant arises. he world's quarter of a million vascular plant species (Heywood and Watson, 1995)
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From page 273... ...
/ vegetative reproduction FIGURE 1 Diagram of basic vascular plant life cycles. Asexual life cycles are indicated with dashed lines.
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From page 274... ...
cites examples from at least seven genera of ferns and points out that it has been known for more than a century that apogamy can be experimentally induced in many other groups (Lang, 1898~. When vascular plants reproduce sexually, the reproductive structures may be borne in many different ways.
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From page 275... ...
Asexual progeny are genetically identical to the individuals that produced them, except for differences caused by somatic mutation. Selfed progeny may differ from their parent as a result of segregation at heterozygous loci, but selfing usually produces far fewer genotypes among offspring than outcrossing.
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From page 276... ...
Thus, the frequency of heterozygotes can be large in asexual populations even if the number of genotypes found is quite small, especially because many apogamous or agamospermous plants are derived from products of hybridization (Manton, 1950; Stebbins, 1950; Grant, 1981~. Agamospermous Crepis in western North America, for example, are polyploids derived from hybridization between different pairs of seven narrowly distributed diploid progenitors (Babcock and Stebbins, 1938)
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From page 277... ...
In both selfing and asexual species, therefore, the genetic structure of their populations may limit their ability to respond adaptively to natural selection. Because the causes of this constraint are the same for both types of uniparental reproduction, it is convenient to refer to it as the uniparental constraint.
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From page 278... ...
As we have just seen, both obligate selfers and obligate asexuals are expected to harbor less genetic variability and to accumulate deleterious mutations more rapidly than sexual outcrossers, which could limit their ability to respond adaptively to environmental change. As a result, we would expect selfing and asexual lineages of plants to be relatively short-lived.
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From page 279... ...
Reproductive Systems and Evolution in Vascular Plants / 279 Failure to recognize the frequency with which self-fertilization has evolved, in particular, has led to many taxonomic mistakes. In Arenaria (Caryophyllaceae)
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From page 280... ...
Nonetheless, open-pollinated, emasculated flowers set as much seed as open-pollinated, unemasculated flowers, demonstrating that self-pollination provides little reproductive assurance in a species where both the capacity for autonomous self-pollination and pollenlimited seed set exist (Eckert and Schaefer, 1998~. The automatic selection advantage of self-fertilization, first pointed out by Fisher (1941)
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From page 281... ...
lust as the forces favoring evolution of self-fertilization, reproductive assurance, and automatic selection are well-known, so also is the primary force opposing its spread, inbreeding depression. Thomas Knight pointed out more than 200 years ago that the selfed progeny of garden peas are less vigorous and fertile than are outcrossed progeny (Knight, 1799~.
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From page 282... ...
Fortunately, it is easy to construct arguments parallel to those for the automatic selection advantage of self-fertilization to show why a similar advantage might accrue to asexual plants in a population of hermaphroditic outcrossers. In a stable population of hermaphrodites, each outcrosser will replace itself, serving once as a seed parent and once as a pollen parent to the outcrossed progeny of another individual.
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From page 283... ...
These observations suggest that cost of outcrossing is a better phrase to describe the automatic selection advantage of selfers and agamosperms relative to sexual outcrossers. The cost of outcrossing arises because selfers and agamosperms can serve as pollen parents of progeny produced by sexual outcrossers, but outcrossers are prevented from serving as pollen parents to the selfed progeny of selfers and the asexual progeny of agamosperms.
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From page 284... ...
Because self-fertilization and asexual reproduction both prevent exchange of genetic material among family lineages, however, the number of genotypes found in populations of predominant selfers or obligate asexuals is usually much smaller than would be found in a population of sexual outcrossers with the same allele frequencies. For similar reasons, surveys have repeatedly shown that a greater proportion of the genetic diversity found in selfing or asexual species is a result of differences among populations than in sexual outcrossers (Brown, 1979; Gottlieb, 1981; Hamrick and Godt, 1989, 1996~.
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From page 285... ...
In the long term, however, differences in the ability of outcrossers, sellers, and asexuals to respond to environmental change and resist the accumulation of deleterious alleles may cause lineages with different reproductive systems to persist for different lengths of time. The smaller number of genotypes in highly selfing and asexual populations may reduce the efficiency with which natural selection can operate, limiting the ability of their populations to respond adaptively to a changing environment the uniparental constraint.
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From page 286... ...
(1998) Frequency-dependent pollen discounting contributes to maintenance of a mixed mating system in the common morning glory Ipomoea purpurea.
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From page 287... ...
(1994) High inbreeding depression, selective interference among loci, and the threshold selfing rate for purging recessive lethal mutation.
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From page 288... ...
(1991) A genetic approach to measuring pollen discounting in natural plant populations.
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