An Extreme Karyotype in an Orthopteran Insect

Abstract

The unique case of an orthopteran acridid of the genus Dichroplus (brachypterous form) with 2n (♂) = 8 chromosomes is described. The karyotype is composed of two pairs of metacentric, one pair of acrocentric and an unequal pair of chromosomes X-Y. The morphological characteristics and the behavior of the three pairs of bivalents and the XY pair can be followed during the prophase and metaphase. All bivalents achieve a segregatory division of the homologues in the first meiotic anaphase. The second meiotic division gives rise to two types of spermatids one with the X chromosome and the other with the Y. The probable mechanisms involved in the production of this extraordinary cytological evolutionary transformation are discussed. I want to express my gratitude to Prof. T. Dobzhansky for the critical reading of the manuscript and to Prof. A. Silveira for having provided the material. I also wish to thank Mr. E. Valdes for technical assistance.