Chromosome elimination is a frequent occurrence in Hemiptera; one of the most interesting cases is that of Sciara. In Sciara coprophila the zygote carries the x chromosomes, one contributed by the egg and two by the spermatozoon (this results from an equational non dis-junction of the maternally derived x chromosome at the second meiotic division in the male following the selective elimination of paternal homologues at the first spermatocyte division). During early cleavage both paternal x chromosomes are eliminated from the somatic cell line of the males while only one is eliminated in the female. In the germ link one paternal x chromosome is eliminated both in the female and in the male, but not until the germ cells have reached their final destination in the gonad. Chromosome elimination may be thought of as a primitive, and in fact crude mechanism of "gene silencing" to be replaced by more subtle devices in the course of evolution. The eliminated chromosomes, or parts of chromosomes, or parts of chromosomes, contain the sexuality genes.
-> Cross section through the pharynx of AscarisEach cell line is committed to a certain number of DNA replication cycles before expressing its specific phenotype: in other words "the program for cell division is a part of the differentiative program of each cell line". We can derive that from a study of development of marine invertebrates. The Ascidian embryo offers unique opportunities to study cell lineage. Indeed, it shows the segregation of the major organ-forming territories occurs before the first cleavage.
Fertilized Ascidian Egg
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