Adaptive structural changes indicate an evolutionary progression towards the open rhabdom in diptera

Abstract

In the present paper, all dipteran rhabdoms examined ultrastructurally so far have been compared. For evaluation of their evolutionary stage the following characteristics and their alternatives have been summarized: Relative length of retinula cells all retinula cells of equal length, no tandem position R7/R8 retinula cell R8 shortened, tandem position R7/R8 Fusion of rhabdomeres rhabdomeres tubularly fused rhabdomeres fused distally only, otherwise separate rhabdomeres completely separate (“open rhabdom”) Position of retinula cell R8 within the ommatidium soma and rhabdomere of R8 both in central position soma in peripheral, rhabdomere in central position Adaptation‐correlated changes in the mutual relationship of rhabdomeres as a function of light intensity rhabdom volume changes with adaptation, but the spatial configuration of the rhabdomeres remains unchanged adaptive changes in rhabdom volume, along with reduction of rhabdomeres during light‐adaptation, results in disruption of their lateral fusion contacts adaptive flexibility of rhabdom structure is very low A tubularly fused rhabdom with 8 retinula cells of equal length and without tandem position of R7/R8 can be regarded as the phylogenetically most primitive type of a dipteran rhabdom. This type of rhabdom may be considered a precursor state for the subsequent development of the typical open rhabdom in Diptera. The listed rhabdom types represent examples of various stages in the evolutionary development of the dipteran eye. However, the data does not allow of any taxonomic interpretation as yet. Morphological and physiological aspects of alterations within the rhabdom are discussed in connection with one another. The cellular mechanism for the “opening” of the rhabdom lies in the light‐dependent structural flexibility of tubularly fused rhabdoms, because reversible fluctuation of the rhabdom volume (increase/reduction) during light/dark‐adaptation may lead to a disruption of the lateral fusion contacts. We consider this fact as the basis for the permanent establishment of an open rhabdom. Copyright © 1990, Wiley Blackwell. All rights reserved