History rather than hybridization determines population structure and adaptation in Populus balsamifera.

Hybridization between species is known to greatly affect their genetic diversity and, therefore, their evolution. Also, within species, there may be genetic clusters between which gene flow is limited, which may impact natural selection. However, few studies have looked simultaneously at the influence of among-species and within-species gene flow. Here, we study the influence of hybridization between Populus balsamifera and Populus trichocarpa on population structure and adaptation in P. balsamifera. We did this by sampling a total of 1517 individuals from across the ranges of these two species, and by genotyping them using a combination of 93 nuclear and 17 cpDNA SNPs. We found that hybridization is mostly limited to the contact zone where the species' distributions overlap. Within P. balsamifera, we found multiple levels of population structure. Interestingly, the border between the Eastern and Central clusters is very sharp, whereas the border between the Central and Western clusters is diffuse. Outlier analysis revealed that three loci associated with the sharp border were also associated with climate. We hypothesize that the observed clusters derive from three refugia during the Pleistocene ice ages. Between the Central and Western clusters, post-glacial long-distance gene flow has led to the diffusion of their border. In the Eastern cluster, we hypothesize that endogenous genomic barriers have developed, leading to the sharp border and a spurious climate association. We conclude that the large-scale genetic structure of P. balsamifera is mostly shaped by historical factors and the influence of interspecific hybridization is limited.

Hybridisation and introgression between Brassica napus and B. rapa in the Netherlands.

We used flow cytometry, chromosome counting and AFLP markers to investigate gene flow from the crop plant oilseed rape, Brassica napus (AACC) to wild B. rapa (AA) in the Netherlands. From 89 B. napus source populations investigated, all near cropping fields or at transhipment sites, only 19 contained a B. rapa population within a 2.5-km radius. During our survey we found only three populations with F1 hybrids (AAC), as recognized by their nine extra chromosomes and by flow cytometry. These hybrids were all collected in mixed populations where the two species grew in close proximity. Populations with F1 hybrids were not close to crops, but instead were located on road verges with highly disturbed soils, in which both species were probably recruited from the soil seed bank. Many plants in the F2, BC1 or higher backcrosses are expected to carry one to eight C chromosomes. However, these plants were not observed among the hybrids. We further investigated introgression with molecular markers (AFLP) and compared sympatric B. rapa populations (near populations of B. napus) with control populations of B. rapa (no B. napus within at least 7 km). We found no difference between sympatric and control populations in the number of C markers in B. rapa, nor did we find that these sympatric populations closely resembled B. napus. Our data show that hybrids occur but also suggest no recent introgression of alleles from the crop plant B. napus into wild B. rapa in the Dutch populations studied.

The effects of inheritance in tetraploids on genetic diversity and population divergence.

Polyploids are traditionally classified into allopolyploids and autopolyploids, based on their evolutionary origin and their disomic or multisomic mode of inheritance. Over the past decade it has become increasingly clear that there is a continuum between disomic and multisomic inheritance, with the rate of tetrasomy differing among species and among chromosomes within species. Here, we use a simple population genetic model to study the impact of the mode of inheritance on the genetic diversity and population divergence of tetraploids. We found that under almost strict disomic inheritance the tetraploid genome is divided into two separate subgenomes, such as found in classical allopolyploids. In those cases, assuming full tetrasomy in the analysis of polyploid genetic data will lead to an important bias in estimates of genetic diversity and population divergence. However, we found that even a low rate of allele exchange between the two subgenomes, at about one event per generation, is sufficient to homogenise the allele frequencies over the subgenomes, and the estimates become essentially unbiased. The inbreeding coefficient F(IS) can then be used to detect whether the estimates of diversity and divergence will be biased when full multisomy is assumed. Finally, we found that different summary statistics for measuring the strength of population differentiation are differentially affected by a deviation from full tetrasomy. Our model results provide several useful guidelines for the analysis of polyploid data, helping researchers to determine when their inferences are biased and which summary statistics to use.

Male sterility in triploid dandelions: asexual females vs. asexual hermaphrodites.

Male reproductive output, pollen in plants and sperm in animals has been shown to constitute a substantial cost for many organisms. In parthenogenetic hermaphrodites, selection is therefore expected to reduce the allocation of resources to male reproductive output. However, sustained production of pollen or sperm has been observed in numerous asexual hermaphrodites. We studied the widespread production of pollen by triploid asexual dandelions, Taraxacum sect. Ruderalia, comparing rare male sterile individuals with pollen producing asexuals. We found that individuals can show plasticity in the production of pollen, but that it is nevertheless possible to distinguish between (facultatively) male sterile asexuals and male fertile asexuals. Based on evidence from genetic markers and crosses, we conclude that the male sterility in asexual dandelions is caused by nuclear genes, in contrast to the cytoplasmically inherited male sterility previously found in sexual dandelions. Male sterile lineages did not produce more seeds per flower head, heavier seeds or seeds that were more viable. However, male sterile plants did produce more seed heads and hence more seeds than pollen producing ones, indicating that they were able to reallocate resources toward seed production. Considering the difference in seed production, it remains puzzling that not more asexual dandelions are male sterile.

Nuclear-cytoplasmic male-sterility in diploid dandelions.

Male-sterility was found in diploid dandelions from two widely separated populations from France, and its inheritance was analysed by crossing a diploid male-sterile dandelion to diploid sexuals and triploid apomicts. Nuclear genetic variation, found in full-sib families, segregated for male-fertility, partial male-sterility, and full male-sterility, and also segregated for small-sized versus normally sized pollen. The crossing results are best explained by a cytoplasmic male-sterility factor in combination with two dominant restorer genes. Involvement of the cytoplasmic male-sterility factor was further investigated by chloroplast haplotyping. Male-sterility was exclusively associated with a rare chloroplast haplotype (designated 16b). This haplotype was found in seven male-sterile plants and one (apparently restored) male-fertile individual but does not occur in 110 co-existing male-fertile plants and not in several hundreds of individuals previously haplotyped. Apomicts with cytoplasmic male sterility were generated in some test crosses. This raises the question as to whether the male sterility found in natural dandelion apomicts, is of cytoplasmic or of nuclear genetic nature. As many breeding systems in Taraxacum are involved in shaping population structure, it will be difficult to predict the evolutionary consequences of nuclear-cytoplasmic male-sterility for this species complex.

Spatial ecological and genetic structure of a mixed population of sexual diploid and apomictic triploid dandelions.

Ecological differentiation is widely seen as an important factor enabling the stable coexistence of closely related plants of different ploidy levels. We studied ecological and genetic differentiation between co-occurring sexual diploid and apomictic triploid Taraxacum section Ruderalia by analysing spatial patterns both in the distribution of cytotypes and in the distribution of genetic variation within and between the cytotypes. A significant relationship between ploidy level and elevation was found. This mode of ecological differentiation however, was not sufficient to explain the significant spatial structure in the distribution of diploids and triploids within the population. Strong congruence was found between the spatial genetic patterns within the diploids and within the triploids. We argue that this congruence is an indication of gene flow between neighbouring plants of different ploidy levels.