Question

Why do the pole cells (the primordial germ cells which give rise to the gametes) form so early in Drosophila melanogaster development?

Answer 1

In Drosophila development the zygote nucleus undergoes several mitotic divisions within the center of the egg. However, cellular membranes do not form until after the thirteenth nuclear division, till then all the nuclei share a common cytoplasm. During the ninth division cycle, about five nuclei reach the surface of the posterior pole of the embryo, while most of the remaining nuclei arrive synchronously at the cortex during interphase of nuclear cycle 10 and then undergo four more divisions at progressively slower rates.The nuclei which reach the posterior of the embryo become enclosed by cell membranes and generate the pole cells that give rise to the gametes of the adult. These are early stages of development in Drosophila insect and these stages of nuclear division are called as syncytial blastoderm, which means that all the cleaved nuclei are contained within a common cytoplasm.

Drosophila is a long germ-band insect, which means there is simultaneous establishment of the anterior and posterior body, including all of the intervening body segments, during the blastoderm stage of insect development. Since both anterior and poterior of insect body is formed in early stage of blastoderm, therefore, pole cells (which are part of posterior growth) form early in Drosophila melanogaster development. This is different from the majority of insects, which undergo short or intermediate germ-band development, where the anterior body segments are are formed initially at the blastoderm stage, and the remaining posterior segments are formed sequentially in posterior growth process.

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