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.