Outcrossing rates in the permanent translocation heterozygote Oenothera biennis L. (Onagraceae)

In species that primarily reproduce asexually, occasional sex can have important evolutionary consequences. Many plant species in the genus Oenothera have a genetic system called permanent translocation heterozygosity (PTH), which renders plants functionally asexual when they self-fertilize. PTH results from reciprocal translocations of chromosomes that suppresses recombination, and a balanced lethal mortality of homozygous gametes that prevents independent assortment of alleles. When PTH plants self-fertilize, the offspring are genetically identical to the parents, but when they outcross they can create new genotypes. Here we sought to determine the rate of outcrossing of natural populations of PTH Oenothera biennis L (Onagraceae). We genotyped 46 parents from 14 populations in Tompkins County, NY, USA, and 754 of their offspring at highly polymorphic microsatellite loci. 99.5% of offspring were genetically identical to their seed parents, whereas 4 offspring out of 754 exhibited allelic mismatches with their seed parent. The estimated outcrossing rate ranged from 0.1 to 0.4%. Our study design also permitted us to estimate mutation rate of microsatellite loci., which ranged from 1.33x10-3 to 3.98x10-3. These results show that O. biennis typically exhibits a functionally asexual genetic system, but rare outcrossing events can generate novel genotypes that may have important evolutionary consequences.