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positive regulator of male germline development. We show that DUO1 is re-
quired for correct male germ cell differentiation including the expression of
key genes required for fertilisation. DUO1 is also necessary for male germ cell
division, and we show that DUO1 is required for the germline expression of
the G2/M regulator AtCycB1;1 and that AtCycB1:1 can partially rescue de-
fective germ cell division in duo1. We further show that the male germline-
restricted expression of DUO1 depends upon positive promoter elements and
not upon a proposed repressor binding site. Thus, DUO1 is a key regulator in
the production of functional sperm cells in flowering plants that has a novel
integrative role linking gametic cell specification and cell cycle progression.
Introduction
The gametes of flowering plants are formed by discrete haploid gametophyte
structures consisting of only a few cells that develop within the diploid repro-
ductive floral organs. During spermatogenesis, each single haploid microspore
divides asymmetrically to produce a larger vegetative cell that eventually gives rise
to the pollen tube and a smaller germ, or generative, cell (reviewed in [1],[2]). In
contrast to germline cells in metozoans [3], angiosperm male germ cells do not
undergo regenerative stem cell divisions, but divide once to form a pair of sperm
cells. These sperm cells are delivered to the embryo sac via the pollen tube, where
they fuse with egg and central cells to produce embryo and endosperm respec-
tively. This process of double fertilization depends upon two functional sperm
cells and is considered one of the major advances in the evolutionary success of
flowering plants. Despite this importance, the molecular mechanisms underlying
many component processes, including the production of both male and female
gametes, remain largely unknown.
Recent transcriptomic analysis of isolated Arabidopsis sperm cells shows that
sperm cells express a distinct and diverse set of genes [4] and there is evidence
for extensive male germ cell gene expression in maize and lily [5],[6]. Several
male germline-specific genes have been characterized in Arabidopsis including
AtMGH3, encoding a histone H3.3 variant [7],[8], AtGEX2, encoding a puta-
tive membrane associated protein [9], and AtGCS1 (HAP2), encoding a sperm
cell surface protein required for fertilisation [10],[11]. Homologues of AtGCS1
are found in many genera [5],[12],[13] that include the green alga Chlamydomo-
nas and the rat malarial parasite Plasmodium berghei, where they are required
for gamete interactions and membrane fusion [13]. Although gene expression
in angiosperm sperm cells is extensive and essential for gamete functions little is
known about its regulation. A transcriptional derepression mechanism, in which
