  Several recent studies show that companion cells in flowering plant
gametophytes relax epigenetic control of transposable elements (TEs) to promote
production of small RNA that presumably assist nearby reproductive cells in
management of TEs. In light of this possibility, a closer look at the timing of
cell division in relation to angiosperm double fertilization is warranted. From
such an analysis, it is conceivable that double fertilization can drive
angiosperm evolution by facilitating crosses between genetically diverse
parents. A key feature of this ability is the order of cell division following
double fertilization, since division of the primary endosperm nucleus prior to
the zygote would produce small RNA capable of identifying TEs and defining
chromatin states in the zygote prior to its entry into S-phase of the cell
cycle. Consequently, crosses leading to increased ploidy or between genetically
diverse parents would yield offspring better capable of managing a diverse
complement of TEs and repetitive DNA. Considering double fertilization in this
regard challenges previous notions that the primary purpose of endosperm is for
improved seed reserve storage and utilization.
