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During double fertilization in angiosperms, sperm nuclear fusions take place shortly after sperm cells fuse with each of the female gametes (egg and central cell). Maruyama et al. (pp. 29-40) found that depletions of BiP1 and BiP2, two Hsp70 proteins in the endoplasmic reticulum of Arabidopsis thaliana, cause nuclear fusion defects in the egg cell and the central cell. A time-lapse confocal microscopy series revealed that nuclear fusion following chromosomal decondensation of the sperm nucleus plays pivotal roles in endosperm development, which is necessary for subsequent seed formation.
The cover shows a confocal image of the nuclear fusion defects in the bip1 bip2 female gametophyte. Magenta signals indicate nuclei of the bip1 bip2 female gametophyte labelled with Histone H2B-tdTomato, while green signals indicate wild-type sperm nuclei expressing a Histone H3 variant HTR10 fused with Clover. Punctate green signal of unfused sperm nucleus (lower signal) is associated with the egg nucleus whereas in the central cell, the unfused sperm nucleus is associated with the two polar nuclei that remained unfused even after fertilization. Image provided by Daisuke Maruyama (Yokohama City University, Japan).
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