The Evolution of Unreduced Gametes
Unreduced gametes are thought to be one of the main mechanisms of polyploid formation in plants, and yet little is known about their evolutionary dynamics. Unreduced gametes can form polyploids through two different pathways. In the simplest form (bilateral pathway), two unreduced gametes unite to form a polyploid, in which case, both female (green) and male (yellow) unreduced gamete production is important. In the two step pathway though, unreduced gametes unite with a normal gamete, forming an intermediate triploid that forms 3x, 2x, and 1x pollen. Through some combination of 3x or 2x gametes from the intermediate triploid going on to unite with either 1x (reduced) or 2x (unreduced) gametes from the parental diploid, polyploids may be formed in this unilateral pathway. Moreover, with this unilateral pathway, polyploids may form despite only one sex contributing unreduced gametes.
But what are the fitness consequences of producing unreduced gametes? Well, they should reduce the fitness of the individual that produces them; their doubled genome content will lead them to contribute to the gamete pool of higher ploidy populations, with little opportunity to for gene flow back into the parental population. Along these lines, unreduced gametes are suspected to occur at relatively low frequencies (typically <2%), which is what we might expect of a deleterious artifact of mistakes in meiosis. So what then can explain the discrepancy between the associated fitness consequence of the production of unreduced gametes and their suspected widespread involvement in polyploid formation? I took a comparative approach to this question, quantifying the frequency of unreduced pollen production using flow cytometry in over 3000 individuals, across 60 populations an 24 species in the Brassicaceae family.
If unreduced gametes are typically deleterious, under mutation-selection balance they should be maintained at low frequencies when selection is strong on sexual reproduction – such as in annuals and outcrossing species – and escape to higher frequencies when selection is weak on sexual reproduction – in perennial, selfing, and asexual species.
We found that unreduced gamete production was consistently higher than previously thought, no individuals produced 0% UG, with some individuals producing extreme values (with frequencies of production up to 85%). Reproductive mode was significantly related the frequency of production across species. As predicted under mutation-selection balance, unreduced gamete production was highest in asexual species, followed by selfing, mixed mating, and then outcrossing species.
Kreiner, J., P. Kron, and B.C. Husband. (2017). Frequency and maintenance of unreduced gametes in natural plant populations: associations with reproductive mode, life history, and genome size. New Phytologist. doi: 10.1111/nph.14423 pdf