Reproduction: Plant Parentage à Trois Thomas Dresselhaus, Mark A. Johnson  Current Biology  Volume 28, Issue 1, Pages R28-R30 (January 2018) DOI: 10.1016/j.cub.2017.11.041 Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 Prevention and occurrence of polyspermy. (A) In animals, thousands of mobile sperm swim towards the egg (ovum). The first sperm penetrating the zona pellucida (ZP) and the vitelline layer fuses and initiates immediate membrane depolarization and slower extracellular matrix modifications (via secretion of cortical granules, blue dots) that prevent fusion of additional sperm. (B) In flowering plants, a single pollen tube (PT) usually releases two immobile sperm cells for double fertilization. One sperm cell fuses with the egg cell, generating the diploid embryo, and the second sperm cell fertilizes the di-haploid central cell forming a triploid endosperm. Accumulation of cell wall material (small blue lines) has been observed around the egg cell soon after in vitro fertilization, which may represent a polyspermy barrier. (C) Using a tetraploid maize line, Grossniklaus now reports that polyspermy of the central cell occurs at a frequency of 2.6% if two PTs release their cargo almost simultaneously. Moreover, a single tetraploid embryo was detected [7]. (D) Nakel et al. report that in Arabidopsis, polyspermy breakdown of the egg cell occurs after polytubey (attraction of at least two pollen tubes) at very low frequencies, generating viable offspring [8]. The fate of the remaining sperm cells illustrated in (C) and (D) is unclear. The percentages of the described fertilization modes in flowering plants are indicated. Current Biology 2018 28, R28-R30DOI: (10.1016/j.cub.2017.11.041) Copyright © 2017 Elsevier Ltd Terms and Conditions