Double fertilization in maize: the two male gametes from a pollen grain have the ability to fuse with egg cells

In flowering plants, two male gametes from a single pollen grain fuse with two female gametes, the egg and central cells, to form the embryo and endosperm, respectively. The question then arises whether the two male gametes fuse randomly with the egg and central cells. We investigated this question using two nearly isogenic maize lines with supernumerary B chromosomes (TB10L18) or without (r-tester). B chromosomes regularly undergo non-disjunction at the second pollen mitosis, producing one sperm cell with zero B chromosomes and one with two. We first confirmed earlier studies showing an excess of transmission of the B chromosomes to the embryo rather than to the endosperm. We then tested the possibility of a directed fertilization. For TB10L18 pollen, we could demonstrate the existence of a size dimorphism between the two sperm cells, correlated to the content in B chromosomes, as detected by fluorescence in situ hybridization (FISH). However, no directed fusion of B chromosome containing sperm to egg cells could be detected when using in vitro fertilization. The absence of directed fusion in vitro could also be demonstrated for control lines. We conclude that both male gametes have the capacity to fuse with the egg cell in maize, although sexual reproduction results in a preferential transmission of supernumerary B chromosomes.