MAYFLIES (EPHEMEROPTERA), THE MOST PRIMITIVE WINGED INSECTS, HAVE TELOTROPHIC MEROISTIC OVARIES

Germ line cell cluster formation in ovarioles of three different stage s, each from a different mayfly species, was studied using ultra-thin serial sectioning. In the analysed ovariole of Cloeon sp., only one li near, zigzag germ line cell cluster was found, consisting of sibling c ells connected by intercellular bridges which represent remnants of pr eceding synchronized mitotic cycles followed by incomplete cytokinesis . A polyfusome stretched through all sibling cells. At the tip of the ovariole, cytokinesis occurred without preceding division of nuclei; t hus, intercellular bridges were lined up but the remaining cytoplasm b etween the bridges had no nuclei. The analysed Siphlonurus armatus vit ellarium contained five oocytes at different stages of development. Ea ch oocyte in the vitellarium was connected via a nutritive cord to the linear cluster of its sibling cells in the terminal trophic chamber. Each cluster had the same architecture as was found in Cloeon. The 3-d imensional arrangement and distribution of closed intercellular bridge s strongly suggest that all five clusters are derived from a single pr imary clone. The position of oocytes within each cluster is random. Ho wever, each oocyte is embraced by follicular or prefollicular cells wh ilst all other sibling cells are enclosed by somatic inner sheath cell s, clearly distinguishable from prefollicular cells. In the analysed o variole of Ephemerella ignita, two small linear clusters were found in the tropharium beside two single cells, two isolated cytoplasmic bags with intercellular bridges but no nuclei, and some degenerating aggre gates. One cluster was still connected to a growing oocyte via a nutri tive cord. In all species the nurse cells remained small and no indica tions of polyploidization were found. We suggest that this ancient and previously unknown telotrophic meroistic ovary has evolved directly f rom panoistic ancestors.