Structure and development of ommatidia in Oncopeltus fasciatus

Abstract

ABSTRACT The structure and development of ommatidia has been examined in Oncopeltus fasciatus (Lygaeidae). Each ommatidium is composed of 18 cells comprising four crystalline cone cells, two primary pigment cells, four secondary pigment cells and eight retinula cells. Ommatidia are of the apposition type and the retinula cells are arranged on the open rhabdomere plan. During the five larval stages the retina grows anteriorly to provide a 12-fold increase in numbers of ommatidia. Grafts of presumptive eye epidermis show that the proliferating anterior border of the developing retina is an area of epidermal cell recruitment rather than a special budding zone. Mosaic retinae were formed by grafting epidermis from near the eye of wild-type donors into the corresponding region of mutant hosts. Ommatidia which developed at the borders of the graft contained mixtures of wild-type and mutant cells in variable and unpredictable combinations. Similar mosaic structures were seen in retinae generated by irradiation at early stages of development. The suggestion that each ommatidium is clonally derived from a single epidermal stem cell is thus disproved. The frequent occurrence of mosaic ommatidia containing only one cell of a different genotype from the rest suggests that the formation of ommatidial clusters follows the main proliferative phase of eye growth. We conclude that cell determination within ommatidia is not connected with lineage but is dependant upon cell position within the developing ommatidium.