Seed yield in red clover is associated with meiotic abnormalities and in tetraploid genotypes also with self-compatibility

Red clover ( Trifolium pratense L.) is a major forage legume crop with diploid and tetraploid cultivars. Seed yield is an important trait in red clover, but the underlying reasons for low seed yield in individual genotypes are not well known. Here we investigated direct associations between seed yield and ovule abortion, male meiotic abnormalities, and pollen quality in well-characterized, diploid and tetraploid red clover genotypes with contrasting seed yields. Ovule development was studied after manual pollinations: in most genotypes, high rates of embryo abortion occurred between 7 and 14 days after pollination. However, the frequencies of developing, underdeveloped and shrivelled ovules were similar in diploid and tetraploid genotypes and were not associated with seed yield. Surprisingly, self-pollination only took place in tetraploid genotypes with high seed yield (83% self-pollination on average). The frequencies of meiotic abnormalities during microsporogenesis were investigated, in addition to pollen viability and pollen germination capacity. Tetraploid genotypes displayed more meiotic abnormalities (17.3% and 7.0% respectively) and lower pollen germination (39% and 63% respectively) than diploids. A negative trend was present between meiotic abnormalities and seed yield. In conclusion, our study reveals that high seed yield was associated with a low degree of abnormalities in male meiosis in diploid and tetraploid genotypes, and with the ability to self-pollinate in the tetraploid genotypes investigated. In certain genotypes, defective male meiosis may be a primary cause of low pollen quality and low seed yield.