Reproductive compensation and selection among viable embryos drive the evolution of polyembryony

Simple polyembryony -- where one gametophyte produces multiple embryos with different sires but the same maternal haplotype -- is common among vascular plants. We develop an infinite-site, forward population genetics model showing that together polyembryony’s two benefits -- "reproductive compensation" achieved by providing a backup for inviable embryos, and the opportunity to favor the fitter of surviving embryos, can favor its evolution. Our model tests how these factors can favor the evolution of polyembryony, and how these underlying benefits of polyembryony shape the genetic load under a range of biological parameters. While these two benefits are difficult to disentangle in nature, we construct variant models of polyembryony that either only include or only exclude the opportunity for reproductive compensation. We find that reproductive compensation strongly favors the evolution of polyembryony, and that polyembryony is favored much more weekly in its absence, suggesting that the benefit of a backup embryo is a major force favoring polyembryony. Remarkably we find nearly identical results in cases in which mutations impact either embryo or post-embryonic fitness (no pleiotropy), and in cases in which mutations have identical fitness effects embryo or post-embryonic fitness (extreme pleiotropy). Finally, we find that the consequences of polyembryony depends on its function – polyembryony results in a decrease in mean embryonic fitness when acting as a mechanism of embryo compensation, and ultimately increases mean embryonic fitness when we exclude this potential benefit.

Methods
The "dataset" was generated by computer simulation in R (code included).