The Terminal Protonephridial Complex of Haplopharynx rostratus (Platyhelminthes,Haplopharyngida)

The terminal protonephridial complex of Haplopharynx rostratus consists of three terminal cells. There are no weirs consisting of ribs connected by a filtration “membrane”, but some cytoplasmic outgrowths into the lumen of the terminal cells. Excretion is by exocytotic vesicles. The terminal cells also contain Golgi complexes and large membrane-bound vacuoles containing electron-dense material. The ciliary bundles (flames) of terminal cells 2 and 3 protrude into the lumen of the centrally located terminal cell I. The complex is surrounded by a sheath containing numerous filaments. The terminal complex of H. rostratus resembles that of the macrostomid Paromalostomum proceracauda, lending support to the view that the two taxa are closely related. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

Differentiation and extragenic transmission of modified gene expression

The present analysis is based on the genemt inDrosophila melanogaster which produces melanotic tumors depending on the genetic and environmental conditions. The cultures were raised under different conditions of temperature and nutrition. When the treatment resulted in at least 40% tumorous flies the cultures were placed in the high category, the others were placed in the low category, and these two groups were compared. The percentage of tumorous flies increases with every successive generation kept under high conditions over seven generations; it decreases with successive generations kept under low conditions. The difference in tumor incidence between flies derived from low cultures and those derived from high cultures is still apparent after seven generations although both groups had received high treatment in each of these seven generations. When high and low treatment alternates per generation the difference due to ancestral treatment is yet preserved. The incidence of tumors is a combination of the effects of direct treatment and ancestral treatment. A case is presented for differentiation being due to the random distribution of particles whose relative number per cell would account for the relative expression of a character like melanotic tumors. In this concept, differentiation is assumed to be the result of the reduction or lack of reduction in a given number of such particles. The present case has a bearing on evolution since, if heritable changes in the extrachromosomal substance were induced by the environment, they would transform the population in a process different from gene mutation.