Reduced and unreduced gametes combine almost freely in a multiploidy system

One of the key modes of plant speciation is polyploidy (duplication of all chromosomes), which can lead to sympatric speciation because it is often associated with limited gene flow and reproductive isolation. To understand the dynamics of sympatric mixed-ploidy systems, we examined the processes operating in them using a general gametic model parameterized by a factorial hybridization experiment. We demonstrate that unreduced gametes combining freely among parents of different ploidy are the core driver of such systems and that the survival of individual gametic combinations drives reproductive isolation. Results of individual crosses deviated slightly from predicted values due to differential gamete acceptance or mentor effects. Proportions of unreduced gametes ranged from 0.001 in diploids to 0.9 in maternal triploids, with strong differences between maternal and paternal gametes. The assumption of free mixing of gametes, which has been made many times in earlier studies, thus gets explicit support by fitting data from a number of crosses into one general formal framework. The strength of the approach also lies in the fact that it can examine whether outcomes of all individual hybridization crosses can be put into one general framework and all its parameter values and their confidence intervals can be obtained using parameter optimization.