Male Germ Unit

The male germ unit (MGU) consists of a stable association between the vegetative nucleus and germ cells (generative cell or sperm cell pair) [Dumas et al., 1984; Mogensen, 1992]. This reproductive unit is transported within the pollen tube and delivered to the embryo sac. Generative and sperm cells are coated in myosin and can be transported along the actin cytoskeleton in vitro or when injected into algal cells. Therefore sperm cell transport may involve an acto-myosin like motility system similar to that described in animal cells. The proposed function of the male germ unit may be to ensure synchrony of the two sperm cell fusion events, with the egg cell and central cell, and the subsequent coordinated development of the resulting zygote and endosperm. In certain species, such as Plumbago, the sperm cells are dimorphic (or unequal) and preferentially fuse with either the egg cell or central cell, further illustrating the tightly controlled cell-cell recognition processes involved in double fertilisation [Russell, 1985].

MGU morphogenesis mutants

In Arabidopsis which possess tricellular pollen, the MGU comprises the associated vegetative nucleus and sperm cell pair and is formed during pollen maturation soon after the generative cell divides to form the two sperm cells. In mature pollen grains the MGU is positioned centrally within the cytoplasm without any obvious polarity.

MGU pollen pictureWe have screened a chemically mutagenised population of Arabidopsis for morphogenesis mutants using the stereotypical MGU visible as a marker in DAPI stained pollen. Two distinct classes of mutants germ unit malformed (gum) and male germ unit displaced (mud) were identified that affect the integrity or the intracellular positioning of the MGU. Mutants in both classes act gametophytically and are fully penetrant such that heterozygous mutants produce 50 % aberrant pollen. Both classes specifically reduce pollen transmission and when homozygous show no sporophytic phenotypes suggesting that GUM and MUD genes are male-specific gametophytic genes required for MGU assembly and positioning. The reduced male transmission of mud and gum mutants supports the hypothesis that the integrity and positioning of the MGU is required for pollen competitive ability and fertility.

Dumas, C., Knox, R.B., McConchie, C.A. and Russell, S.D. (1984). Emerging physiological concepts in fertilisation. What's News in Plant Physiology, 15, 17-20.

Mogensen, H.L. (1992) The male germ unit: concept, composition and signification. Int. Rev Cytol. 140, 129-147.

Russell, S.D. (1985) Preferential fertilisation in Plumbago: ultrastructural evidence for gamete level recognition in an angiosperm. Proc. Natl. Acad. Sci. USA 82, 6129

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