A pseudo-multiple sex-chromosome mechanism in an indian gryllid

Summary A sample of 55 testes of the Indian gryllidEuscyrtus sp. comprised 33 with 2n=19 chromosomes, 18 with 2n=20, two with 2n=21, and one with 2n=19 and 2n=20 in different follicles of the testis: the extra chromosomes in 20- and 21-chromosome individuals are, by definition, supernumeraries. They are transitorily heteropycnotic during meiosis and presumably genetically inactive. Their anaphase segregation is correlated with that of the X-chromosome (strongly, positively heteropycnotic) to an extent possibly determined by the degree to which they exhibit heteropycnosis.A precocious separation of the components of a pair of autosomes (P-chromosomes) produces a pseudo-multiple sex-determining mechanism through the precession of one and the X to one pole, the second to the other pole. Actually, this species ofEuscyrtus is, like the other cytologically known members of the genus, an XXXO type.The precocious movement of the P-chromosomes is conditioned by a pre-metaphase stretch, which is of extremely short duration but initiated with such violence as to culminate in the premature disjunction of these chromosomes and these only.The pre-metaphase stretch, previously established as a taxonomic attribute of the phasmid, mantid, and blattid components of the orthopteroid complex, is thus found to be a characteristic feature in the cytology of this member of the gryllid component: this is no more than a reflection of their common ancestry.Contribution number 86, Division of Forest Biology, Science Service, Department of Agriculture, Ottawa.