New advances and future directions in plant polyspermy

Abstract

Plants have evolved a battery of mechanisms that potentially act as polyspermy barriers. Supernumerary sperm fusion to one egg cell has consequently long remained a hypothetical concept. The recent discovery that polyspermy in flowering plants is not lethal but generates viable triploid plants is a game changer affecting the field of developmental biology, evolution, and plant breeding. The establishment of protocols to artificially induce polyspermy together with the development of a high-throughput assay to identify and trace polyspermic events in planta now provide powerful tools to unravel mechanisms of polyspermy regulation. These achievements are likely to open new avenues for animal polyspermy research as well, where forward genetic approaches are hampered by the fatal outcome of supernumerary sperm fusion.

Keywords: hybridization; plant; polyploidization; polyspermy.

Figure 1. Plant polyploidization scenarios.

(a) During regular fertilization, a haploid sperm (color) fertilizes a haploid egg and a diploid central cell to give rise to a diploid embryo and triploid endosperm, respectively. (b and c) Cell-cycle defects can result in somatic doubling (b) or unreduced gametes (c). Whilethe first scenario equally affects both sexes, unreduced gametes are considered mainlyto be formed in the male germline, resulting in an unbalanced female-male genome contribution in the seed. (d and e) It is currently unclear whether polyspermy introduces extra paternal copies into both gametes (d) or whether selective egg cell polyploidization is possible (e).