CYTOLOGY AND PARTHENOGENESIS OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS NORT. (HYMENOPTERA: CEPHIDAE)

Abstract

A chromosome complement of 18 was established for the female of Cephus cinctus Nort. by counts of 18 elements at oogonial metaphase and nine bivalents at pachytene. The male has a haploid set of nine chromosomes on the basis of counts at spermatogonial metaphase and at meiotic metaphases I and II. Male haploidy was confirmed by the fact that the male karyotype comprises one member of each morphologically identifiable pair present in the female. Spermatogenesis is characterized by lack of synapsis and complete abortion of the first meiotic division. Formation of a monopolar spindle at first meiotic metaphase is discussed. A cytological comparison of thelyotokous and arrhenotokous strains of C. cinctus failed to reveal any difference in chromosome number or in chromosome morphology. Cephus cinctus reproduces largely by arrhenotoky, the population normally being bisexual; the males develop from unfertilized eggs, most of the females arising from fertilized eggs; azygous, or impaternate, females occur sporadically. From the bisexual population a thelyotokous strain has arisen that is not distinguishable by cytological comparison; the females of both arrhenotokous and thelyotokous races are diploid; no diploid males were found. The haplo-diploid relationship of male to female was confirmed; male haploidy has become established by adaptive modifications in meiosis, as indicated in the cytological details given.