Structure and function of the fused rhabdom

Summary

This paper considers the functional significance of fused rhabdoms. Since all rhabdomeres are joined tightly together, the possibility of optical and electrical coupling between retinula cells is greatly enhanced. We study the extent and consequences of this coupling in order to understand the functional significance of fused rhabdoms. Our methods include both theory and intracellular recordings. The results are as follows:

Optical Coupling. Because rhabdomeres of different spectral types are fused into a common light guide, the absorption properties of each influence the manner in which light is transmitted along the composite rhabdom structure.

• 1. Each rhabdomere acts as if it were an absorption filter in front of all others, i.e. rhabdomeres function as lateral absorption filters (Fig. 4).

• 2. As a consequence of this filtering, the shape of the spectral sensitivity curve for each retinula cell is approximately independent of the amount of light it absorbs, i.e. independent of the rhabdomere's length and concentration of photopigment (Fig. 7). This is in direct contrast to the retinula cells of fly that have spectral sensitivity curves which become progressively flatter as more light is absorbed (Snyder and Pask, 1973). In other words, the flattening of curves by self absorption is prevented by optical coupling.

• 3. Thus, one functional advantage of the fused rhabdom (due to optical coupling) is that each retinula cell can have a high absolute sensitivity while preserving its spectral identity (narrow spectral sensitivity curves). (Compare Fig. 5 to Fig. 6.) Thus the same receptors can operate in a high sensitivity and in a colour vision system (cf. vertebrate rods and cones).Since all spectral cell types are together in one rhabdom, the animal can have hue discrimination in a small field of view (fine grain colour vision). Thus an individual ommatidium has the potential for providing excellent spectral discrimination.

• 4. If two cells have photopigments with absorption maxima close together, the maxima of their spectral sensitivity curves are moved further apart (Fig. 8).

• 5. In the absence of electrical coupling polarization sensitivity (PS) can depend dramatically on wavelength. The spectral composition of the rhabdom, in addition to the direction of the microvilli, profoundly influences the polarization sensitivity vs. wavelength PS (λ) curves of individual retinula cells. This is shown theoretically for the worker bee rhabdom (Fig. 10) where (a) there is a pronounced difference in PS (λ) between cells with orthogonal microvilli and (b) green retinula cells show a large PS in the green while the UV cells show a much smaller PS in the UV (Fig. 13).