The meiotic process inmantis religiosa L. males

Summary

• 1. Mantis religiosa has a diploid chromosome number of 27 in males, there being 12 pairs of autosomes and an X₁X₂Y sex-determining trivalent at first meiosis.

• 2. As in most other mantid species which have been studied cytologically, there is no diplotene stage in meiotic prophase. Pachytene is followed by a ‘prometaphase stretch’ stage, when persistent points of association between homologous chromosomes are revealed. Most bivalents remain associated in one arm only at metaphase, though bivalents with associations in both arms on either side of the centromere also occur.

• 3. No failure of pairing is observable at pachytene, but cells with unassociated autosomes or sex-chromosomes can be seen at metaphase. Univalent sex-chromosomes are present three times as frequently as univalent autosomes, and X₂ occurs as a univalent twice as often as X₁. Cells containing univalents remain ‘blocked’ at metaphase and ultimately degenerate; this may explain the absence of supernumerary chromosomes in manticls, a notable distinction between these animals and. short-horned grasshoppers.

• 4. Malorientation of the sex-trivalent occurs in less than 2% of first meiotic metaphases. It leads to non-disjunction of the sex-chromosomes and to peculiar and characteristic chromosome, bridges joining sister second spermatocytes.

• 5. In spite of the absence of a diplotene stage, there are strong grounds for supposing that autosomes and sex-chromosomes are associated by chiasmata at first meiotic metaphase.

• 6. It has been suggested by White that X₁X₂Y rnantid species have a monophyletic origin from a primitive XO species in which translocation took place between the original X-chromosome and one of a pair of autosomes. Whereas in other X₁X₂Y mantids the relative sizes and mode of association of the three sex-chromosomes indicate that considerable evolutionary change has taken place since the trivalent sex-determining mechanism was first established, there is evidence that inM. religiosa we are able to see the sex-chromosomes in their ancestral form and meiotic behaviour.