Advantage Of Asexual Reproduction Research Articles

Evolution and biogeography of alpine apomictic plants

AbstractAsexual organisms often occupy larger and more northern distribution areas than their sexual relatives, and tend to colonize more frequently previously glaciated areas. These phenomena, summarized under the term "geographical partheno ‐ genesis", seem to imply a short‐term advantage of asexual reproduction. Alpine biota, for instance, with their short growing seasons, cold climates and climatic histories shaped by glaciations are predisposed for the evolution and spread of apomictic plants. However, patterns and causes of geographical parthenogenesis for alpine species remain elusive, and for many alpine taxa the mode of reproduction is only poorly characterized. A survey of apomictic species in the European Alps presents some examples and case studies on geographical parthenogenesis. The distribution pattern may be explained by better colonizing abilities of asexual organisms because of uniparental reproduction. Apomixis has been advantageous for re‐colonization of the Alps after the retreat of glaciers, because of their ability to rapidly found populations via single individuals (Baker's Law). Asexual organisms also may perform better in diverse and narrow ecological niches. The distributional superiority of asexuals has also been attributed to indirect advantages of hybridity and/or polyploidy. Sexual hybrids or polyploids, however, when compared to sexual diploid progenitors, do not show patterns of geographical parthenogenesis. However, in the Alps, climatic oscillations may have triggered range fluctuations of species and breakdown of crossing barriers. Interspecific hybridization and polyploidy may have spurred recurrent origins of asexuality, while decreasing the fitness of sexual progenitor species. Biogeographical history in combination with the intrinsic advantages of apomixis may explain best the relative success of alpine apomictic plants.

Maintenance of a genetic polymorphism by fluctuating selection on sex-limited traits

Abstract Fluctuating selection is often thought to be ineffective in maintaining a genetic polymorphism except when generations overlap, for example when a seed bank causes a storage effect. Here, I demonstrate that fluctuating selection on sex-limited traits automatically includes such a ‘storage effect’ because sex-limited alleles are shielded from selection in the sex where they are not expressed. With analytical calculations and numerical simulations I show that fluctuating selection can maintain a genetic polymorphism in sex-limited traits. Such a protected polymorphism can reduce the cost of sex when female-limited traits are involved. But, this effect will probably be small compared to the two-fold advantage of asexual reproduction unless many polymorphic loci interact or exceptionally strong environmental fluctuations are present. It is argued that genetic polymorphisms maintained by fluctuating selection on sex-limited traits may partly explain the large genetic variance of traits under strong sexual selection.

Evolution and Stasis in Plant‐‐Pathogen Associations

The impact of disease on plant populations may change as a result of coevolutionary interactions. The Red Queen hypothesis suggests that parasites provide a selective advantage to host sexual recombination sufficient to explain it persistence despite the twofold advantage of asexual reproduction. The constant production of resistant host genotypes by sexual recombination favors parasite genotypes that can overcome host resistance, resulting in a coevolutionary arms race. Systemic fungal parasites differ from nonsystemic fungi in ways that affect plant—pathogen interactions. Many systemic fungal parasites castrate their host plants, which reduces the potential for production of genetically variable, resistant progeny. Moreover, host genotypes susceptible to infection by the maternal pathogen are perpetuated through parasite—induced asexual reproduction or enforced selfing. Grasses in the genus Danthonia infected by Atkinsonella hypoxylon represent a model system for investigating evolution and stasis in plant—pathogen associations. Infected plants, unlike uninfected plants, reproduce only by self—fertilized seed through which the fungus is vertically transmitted. Other similar associations exist, suggesting that parasites can often enforce genetic uniformity within host populations.

A Spatial Mechanism for the Evolution and Maintenance of Sexual Reproduction

In this paper we discuss a spatial mechanism for the evolution and maintenance of sexual reproduction. We consider three related models in which sexual reproduction is maintained by parasitism despite the twofold reproductive advantage to pathenogenic females. These models take into account the fact that the populations are spatially extended and that the effective local population size is relatively small. They do not rely on the deterministic cycling of genotypes but on the dynamically produced local stochastic genetic variation. The primary model is a probabilistic cellular automaton. In this, for a wide range of parasite mutation rates, the parasites maintain a spatially genetically heterogeneous population of sexuals and this allows the sexuals to overcome the twofold advantage of asexuals because parasites and their adaptation are much less effective in a stochastic spatial genetic structure. We also consider the case where the sexuality rate S (the proportion of the time the host breeds sexually) is slowly evolving. With such slow mutation, we find that both sexual (S=1) and asexual (S=0) populations are evolutionarily stable. We examine two other models which allow us to consider the mathematical conditions under which the advantage of this spatial genetic structure overcomes the twofold advantage of asexual reproduction.

SUBTERRANEAN SPOROPHYTIC GEMMAE IN MOONWORT FERNS, BOTRYCHIUM SUBGENUS BOTRYCHIUM

Underground gemmae in Botrychium subgenus Botrychium are described as the first report of such structures in ferns. Gemmae are spherical units 0.5–1 mm in diameter produced on the stem, where they originate in meristematic regions of the outer cortex. At maturity they contain an embedded apical cell and may contain a mycorrhizal fungus. Upon abscission from the stem they remain subterranean and may undergo growth and development into completely differentiated sporophytes in a pattern similar to that of development of sexually produced embryos. In addition to the diploid B. campestre, gemmae occur in the tetraploids B. minganense, B. echo, and a new species as yet undescribed. These may be allotetraploids of which B. campestre is one parent. The ecological significance of reproduction by gemmae may lie in the advantages of asexual reproduction in the dry prairie habitat of B. campestre.