Beyond GNU/Linux: GNU/Egg and GNU/Embryo

Something to think about…

Maternal effect loci essential for mitosis

gnu, a mutation specifically affecting early embryogenesis

The mutation gnu identifies a gene whose product is needed for nuclear division during early development. Females homozygous for gnu lay eggs that develop giant nuclei as a result of continued DNA replication, in the absence of chromosome segregation and nuclear division (Freeman et al. 1986). Fertilization of GNU eggs is not required for the giant nuclei to develop, contrasting with wild-type eggs in which fertilization is required before any further development can take place (Freeman & Glover, 1987). Whether or not the GNU egg is fertilized, any of the four products of female meiosis, the three polar bodies or female pronucleus, can participate in DNA synthesis to give giant nuclei. By marking the paternal genome with a bacterial gene, it has been shown that if the GNU egg is fertilized, then the DNA derived from the male pronucleus also undergoes DNA replication in fertilized GNU embryos. This same approach has also been used to demonstrate that DNA of the paternal genome fails to replicate in fertilized eggs of mothers homozygous for the maternal haploid mutation, mh. Fertilization is required to trigger the development of eggs laid by females homozygous for mh, but syngamy does not occur and the female pronucleus undergoes multiple rounds of haploid nuclear division. However, if females are homozygous for both gnu and mh, then both maternal and paternal genomes are replicated and, furthermore, the embryos develop giant nuclei (Freeman & Glover, 1987). It seems that somehow, the GNU cytoplasm is lifting the repression of DNA synthesis that normally occurs following the completion of meiosis until the fusion of the male and female pronuclei has taken place. The gene therefore appears to play a role in the correct establishment of coordinated DNA replication and mitosis in zygotic development.

Uncoupling of mitotic cycles from DNA replication in the early embryo

One of the striking features of GNU embryos is that although nuclear division does not take place, centrosomes continue to replicate (Fig. 1). Normally, each wild-type interphase nucleus is associated with a single centrosome that replicates, giving rise to two daughter centrosomes. These migrate to opposite sides of the nucleus in prometaphase to nucleate the microtubules of a new spindle. The centrosomes of GNU embryos are dissociated from nuclei and do not function in the formation of mitotic spindles per se, but are capable of nucleating asters of microtubles. They increase in number and migrate to the cortex of the developing GNU embryo as they would in the wild-type, indicating that although the nuclear division cycle has been disrupted, the centrosome cycle continues independently. Spindlelike structures can be seen when the embryos become necrotic and the giant nuclei break down to yield fragments of chromatin, which can organize microtubules. This would seem to indicate that there is nothing inherently wrong with the mitotic apparatus of GNU embryos per se (Freeman et al. 1986). The GNU phenotype indicates that there is a centrosomal component of the cell cycle that is capable of running independently of the nuclear division cycle.

–David M. Glover, Mitosis in Drosophila
   http://jcs.biologists.org/cgi/reprint/92/2/137.pdf