Genetic and environmental determinants of unreduced gamete production in Brassica napus, Sinapis arvensis and their hybrids

Abstract

Unreduced gametes, sperm or egg cells with the somatic chromosome number, are an important mechanism of polyploid formation and gene flow between heteroploid plants. The meiotic processes leading to unreduced gamete formation are well documented, but the relative influence of environmental and genetic factors on the frequency of unreduced gametes remain largely untested. Furthermore, direct estimates of unreduced gametes based on DNA content are technically challenging and, hence, uncommon. Here, we use flow cytometry to measure the contribution of genetic (hybridization) and environmental (nutrient limitation, wounding) changes to unreduced male gamete production in Brassica napus, Sinapis arvensis and two hybrid lines. Treatments were applied to greenhouse grown plants in a random factorial design, with pollen sampled at two time intervals. Overall, the frequency of unreduced gametes averaged 0.59% (range 0.06-2.17%), plus a single outlier with 27%. Backcrossed hybrids had 39 to 75% higher unreduced gamete production than parental genotypes, averaged across all treatments, although the statistical significance of these differences depended on sampling period and wounding treatment. Unreduced gamete frequencies were higher for the second sampling period than the first. There were no direct effects of wounding or nutrient regime. Our results indicate that both genetic and environmental factors can induce increased unreduced gametes, highlighting the potential importance of environmental heterogeneity and genetic composition of populations in driving polyploid evolution.

Frequency and maintenance of unreduced gametes in natural plant populations: associations with reproductive mode, life history and genome size

“…Although we cannot exclude the possibility that this is a methodological artifact (background noise inflating very low estimates; Kron & Husband, ), it raises the possibility that 2 n gamete frequencies in many natural populations may best be described as continuous, at least in Brassicaceae. Although previous studies have shown a relationship between flow cytometric 2 n gamete estimates and measures of large pollen frequency (Van Tuyl et al ., ) or average pollen size (Sora et al ., ), future studies comparing these methods more directly could shed light on how methodology may have influenced our understanding of 2 n gamete frequency and variation in natural populations.…”

“…To minimize the error for estimates of rare (2C) events, we excluded samples with fewer than 3000 total nuclei. We corrected for possible somatic tissue contamination using the method described in Sora et al (2016), and we performed our analyses (see Life history, reproductive and genomic associations with 2n gamete production) using both uncorrected and corrected estimates.…”

Bottlenecks and inbreeding depression in autotetraploids

“…Given that transitions to higher ploidy often involve relatively few founding individuals (Ramsey and Schemske ; Sora et al. ), additional selective advantages of selfing may also be likely considering the reproductive assurance provided by selfing (Schoen et al. ; Goldberg and Igic ).…”

“…Even in the absence of this mechanism, the formation of unreduced gametes may be frequent enough to permit continual introduction of new genetic material into establishing polyploid populations (Soltis and Soltis ; Sora et al. ). Between‐cytotype migration could affect our results in a few interesting ways.…”

“…This results from the decrease in the number of segregating mutations due to bottleneck effects combined with the masking of partially recessive mutations in several heterozygous genotypes. Given that transitions to higher ploidy often involve relatively few founding individuals (Ramsey and Schemske 1998;Sora et al 2016), additional selective advantages of selfing may also be likely considering the reproductive assurance provided by selfing Goldberg and Igic 2012). This advantage potentially insulates selfing populations from minority cytotype exclusion when neopolyploids overlap geographically with diploid relatives (Levin 1975;Rausch and Morgan 2005).…”