History of the Germ Line in Male and Female Thrips

Abstract

The history of the germ cells (GCs) in Thysanoptera is reviewed from their origin at the posterior end of the egg and includes their proliferation and differentiation in embryos, larvae, pupae and adults, the transfer of sperm from male to female, the allocation of sperm to eggs, and oviposition. Clonal proliferation of oogonia in immatures suggests the ovarioles of thrips to be ‘neopanoistic’ and to probably derive by reduction from the polytrophic ovarioles of a psocopteroid ancestor, perhaps as an adaptation to small adult body size. Each testis in immature terebrantians contains a single cyst of developing GCs while those of tubuliferans contain many. Because most thrips are arrhenotokous and have a haploid male germ line, male meiosis is aberrant: meiosis I is mitotic and II is reduced to the ejection of a chromatin mass from each secondary spermatocyte. The unique development and structure of the sperm suggest that they evolved from an ancestral gamete having two 9 + 2 or 9 + 9 + 2 axonemes. Sperm transfer is probably direct in phlaeothripine phlaeothripids but seems to be by spermatophore in at least some aeolothripids, thripids and idolothripine phlaeothripids. Details are compared with those of related insects, their evolution is addressed and suggestions are made for future research.