Evolution of Eye Morphology and Rhodopsin Expression in the Drosophila melanogaster Species Subgroup

A striking diversity of compound eye size and shape has evolved among insects. The number of ommatidia and their size are major determinants of the visual sensitivity and acuity of the compound eye. Each ommatidium is composed of eight photoreceptor cells that facilitate the discrimination of different colours via the expression of various light sensitive Rhodopsin proteins. It follows that variation in eye size, shape, and opsin composition is likely to directly influence vision. We analyzed variation in these three traits in D. melanogaster, D. simulans and D. mauritiana. We show that D. mauritiana generally has larger eyes than its sibling species, which is due to a combination of larger ommatidia and more ommatidia. In addition, intra- and inter-specific differences in eye size among D. simulans and D. melanogaster strains are mainly caused by variation in ommatidia number. By applying a geometric morphometrics approach to assess whether the formation of larger eyes influences other parts of the head capsule, we found that an increase in eye size is associated with a reduction in the adjacent face cuticle. Our shape analysis also demonstrates that D. mauritiana eyes are specifically enlarged in the dorsal region. Intriguingly, this dorsal enlargement is associated with enhanced expression of rhodopsin 3 in D. mauritiana. In summary, our data suggests that the morphology and functional properties of the compound eyes vary considerably within and among these closely related Drosophila species and may be part of coordinated morphological changes affecting the head capsule.

昆虫类群中演化出了极为多样的复眼（compound eye）尺寸与形态结构。小眼（ommatidium，复数形式为ommatidia）的数量与大小是决定复眼视觉灵敏度与分辨率的核心因素。每个小眼由8个感光细胞组成，这些细胞通过表达多种感光视紫红质（rhodopsin）蛋白，协助机体区分不同色彩。由此可见，复眼尺寸、形态以及视蛋白（opsin）组成的变异，很可能会直接影响视觉功能。
我们针对黑腹果蝇（D. melanogaster）、拟果蝇（D. simulans）以及毛里求斯果蝇（D. mauritiana）的上述三项性状展开了变异分析。研究结果显示，毛里求斯果蝇的复眼整体大于其近缘姊妹物种，这一现象源于小眼尺寸更大且小眼数量更多的共同作用。此外，拟果蝇与黑腹果蝇品系间的眼尺寸种内与种间差异，主要由小眼数量的变异所导致。
我们采用几何形态测量法（geometric morphometrics）分析了更大复眼的形成是否会影响头壳（head capsule）的其他区域，结果发现复眼尺寸增大与相邻颜面表皮（face cuticle）的缩减存在显著关联。形态分析还表明，毛里求斯果蝇的复眼在背侧区域存在特异性增大。有趣的是，这种背侧增大与毛里求斯果蝇体内视紫红质3（rhodopsin 3）的表达上调存在相关性。
综上，我们的数据表明，在这些近缘果蝇物种内部及物种间，复眼的形态结构与功能特性存在显著差异，且这一差异可能属于影响头壳的协同形态变化的一部分。