Rare Sexual Events Research Articles

Population genomics of an outbreak of the potato late blight pathogen, Phytophthora infestans, reveals both clonality and high genotypic diversity.

SummaryAn outbreak of the potato late blight pathogen Phytophthora infestans in Denmark was characterized in order to resolve the population structure and determine to what extent sexual reproduction was occurring. A standard set of microsatellite simple sequence repeats (SSRs) and single nucleotide polymorphism (SNP) markers generated using restriction site‐associated DNA sequencing (RAD‐seq) were employed in parallel. A total of 83 individuals, isolated from seven different potato fields in 2014, were analysed together with five Danish whole‐genome sequenced isolates, as well as two Mexican individuals used as an outgroup. From a filtered dataset of 55 288 SNPs, population genomics analyses revealed no sign of recombination, implying clonality. In spite of this, multilocus genotypes were unique to individual potato fields, with little evidence of gene flow between fields. Ploidy analysis performed on the SNPs dataset indicated that the majority of isolates were diploid. These contradictory results with clonality and high genotypic diversity may suggest that rare sexual events likely still contribute to the population. Comparison of the results generated by SSRs vs SNPs data indicated that large marker sets, generated by RAD‐seq, may be advised going forward, as it provides a higher level of genetic discrimination than SSRs.

Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitment.

In dioecious species with both sexual and asexual reproduction, the spatial distribution of individual clones affects the potential for sexual reproduction and local adaptation. The seaweed Fucus radicans, endemic to the Baltic Sea, has separate sexes, but new attached thalli may also form asexually. We mapped the spatial distribution of clones (multilocus genotypes, MLGs) over macrogeographic (>500km) and microgeographic (<100m) scales in the Baltic Sea to assess the relationship between clonal spatial structure, sexual recruitment, and the potential for natural selection. Sexual recruitment was predominant in some areas, while in others asexual recruitment dominated. Where clones of both sexes were locally intermingled, sexual recruitment was nevertheless low. In some highly clonal populations, the sex ratio was strongly skewed due to dominance of one or a few clones of the same sex. The two largest clones (one female and one male) were distributed over 100-550km of coast and accompanied by small and local MLGs formed by somatic mutations and differing by 1-2 mutations from the large clones. Rare sexual events, occasional long-distance migration, and somatic mutations contribute new genotypic variation potentially available to natural selection. However, dominance of a few very large (and presumably old) clones over extensive spatial and temporal scales suggested that either these have superior traits or natural selection has only been marginally involved in the structuring of genotypes.

Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

The co-occurrence of apomixis (asexual reproduction) and polyploidy in plants has been the subject of debate in regard to the origin and evolution of asexuality. In recent years, polyploidy has been postulated as a maintenance and stabilization factor rather than as a source of apomixis origin. It is assumed polyploidy facilitates the compensation for mutation accumulation, and hence, the rare occurrence of diploid apomixis indirectly supports this finding. Nevertheless, diploid apomicts exist and are successful, especially in the genus Boechera. While comparing phenotypic traits, fitness-related traits and apomixis penetrance between both diploid and triploid apomicts in the genus Boechera, it was expected to find trait variance that can be attributed to ploidy. Surprisingly, little trait variation could be assigned to ploidy, but rather trait variations were mainly genotype-specific. Additionally, it is shown that paternal contribution is very important for trait success, even though all offspring are genetically identical to the mother plant. This harbors implications for the introduction of apomixis into crop plants, considering the effects of paternal contribution during asexual reproduction. Nevertheless, polyploidy is an efficient way to buffer deleterious mutations, but the flexibility of diploid apomicts of the genus Boechera for rare sexual events contributes to their success in nature.

A Histocompatibility Analysis of Inheritance in the Unisexual Fish Poeciliopsis 2 monacha-lucida

Poeciliopsis 2 monacha-lucida is a triploid unisexual fish species which reproduces by gynogenesis. Spleen grafts between genetic lines descendent from single, wild-caught females demonstrated that natural populations are comprised of several histocompatibility clones. Histoincompatible clones are characterized by acute allograft reactions. Grafts within genetic lines, i.e. mother to daughter, daughter to mother and between sibs were all accepted as were grafts between histocompatible lines. Inheritance in P. 2 monacha-lucida is strictly clonal. Certain pairs of lines are characterized by unidirectional histocompatibility; i.e., a graft from line A to B is accepted, but the reciprocal graft, line B to A, is rejected. This suggests one clone was derived from the other perhaps by a mutation, a rare gametic fusion or a rare recombinational event. P. 2 monacha-lucida is like another gynogenetic unisexual fish Poecilia formosa in that both species are polyclonal (Kallman, 1962b). These two gynogenetic fishes are a contrast to the thelytokous unisexual lizard Cnemidophorus tesselatus in that populations of the latter usually consist of a single clone (Maslin, 1967). Several factors must affect the clonal structure of natural populations. Selection can act to perpetuate or eliminate clones, whereas stochastic factors always reduce the number of clones. New clones can arise by mutation, rare sexual events such as gametic fusion and recombination, deletion and recurring hybridization between parental species. The preliminary nature of the studies reported to date preclude conclusive judgements as to which factors are most important. It is clearly possible, however, that mutation is not the sole source of new clones and that selection plays some role in main