Effects of floral neighborhood on seed set and degree of outbreeding in a high-alpine cushion plant.

Plants flowering together may influence each other's pollination and fecundity over a range of physical distances. Their effects on one another can be competitive, neutral, or facilitative. We manipulated the floral neighborhood of the high-alpine cushion plant Eritrichium nanum in the Swiss Alps and measured the effects of co-flowering neighbors on both the number of seeds produced and the degree of inbreeding and outbreeding in the offspring, as deduced from nuclear microsatellite markers. Seed set of E. nanum did not vary significantly with the presence or absence of two Saxifraga species growing as near neighbors, but it was higher in E. nanum cushions growing at low conspecific density than in those growing at high density. In addition, floral neighborhood had no detectable effect on the degree of selfing of E. nanum, but seeds from cushions growing at low conspecific density were more highly outbred than seeds from cushions at high density. Thus, there was no evidence of either competition or facilitation between E. nanum and Saxifraga spp. as mediated by pollinators at the spatial scale of our experimental manipulation. In contrast, the greater fecundity of E. nanum cushions at low density was consistent with reduced intraspecific competition for pollinators and might also represent a beneficial effect of highly outbred seeds as brought about by more long-distance pollinator flights under low-density conditions.

Lower selfing rate at higher altitudes in the alpine plant Eritrichium nanum (Boraginaceae).

A general hypothesis on mating patterns in alpine plants states that self-fertilization should increase with increasing altitude as a result of pollinator limitation at higher altitudes. However, realized selfing rates under natural conditions, as based on genetic progeny analysis, have not yet been determined for any alpine species across altitude. We therefore assessed the realized selfing rates in about 100 open-pollinated families of the high-alpine cushion plant Eritrichium nanum, sampled along an altitudinal gradient in the Swiss Alps, by using progeny analysis based on six microsatellites. In marked contrast to the general hypothesis, realized selfing rates in E. nanum significantly decreased with increasing altitude, and only progenies from low altitudes were predominantly selfed. However, the higher selfing rates of individuals at lower altitudes could have been caused by unfavorable weather conditions during early growing season when low-elevation plants flowered. In summary, our results on selfing rates in an alpine plant across altitude as well as the results of other studies using experimental hand-pollinations and/or population genetic methods generally do not support the expectation of higher selfing rates at higher altitudes. We therefore ask for further critical examination of realized mating systems in alpine plants.

Allelic configuration and polysomic inheritance of highly variable microsatellites in tetraploid gynodioecious Thymus praecox agg.

Polyploidy plays a pivotal role in plant evolution. However, polyploids with polysomic inheritance have hitherto been severely underrepresented in plant population genetic studies, mainly due to a lack of appropriate molecular genetic markers. Here we report the establishment and experimental validation of six fully informative microsatellite markers in tetraploid gynodioecious Thymus praecox agg. Sequence data of 150 microsatellite alleles and their flanking regions revealed high variation, which may be characteristic for polyploids with a reticulate evolutionary history. Understanding the patterns of mutation (indels and substitutions) in microsatellite flanking-sequences was a prerequisite for the development of co-dominant markers for fragment analyses. Allelic segregation patterns among progeny arrays from ten test crosses revealed tetrasomic inheritance in T. praecox agg. No evidence of frequent double reduction was detected. Polymerase chain reaction (PCR) based dosage effects allowed for precise assignment of allelic configuration at all six microsatellite loci. The quantification of allele copy numbers in PCR was verified by comparisons of observed and expected gametic allele frequencies and heterozygosities in test crosses. Our study illustrates how PCR based markers can provide reliable estimates of heterozygosity and, thus, powerful tools for breeding system and population genetic analyses in polyploid organisms.