The making of an eye: structural and functional diversity of stemmata.
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1
University of Cincinnati, Biological Sciences, United States
Eyes are extremely diverse sensory organs, yet there is mounting evidence that this diversity is largely built by a set of highly conserved molecular developmental mechanisms. Stemmata (the larval eyes of holometabolous insects) are an excellent example of such diversity, as they can range from very simple light detectors, to elaborate visual systems with image-forming eyes.
Among the most sophisticated examples of stemmata are the principal eyes of the diving beetle larvae of Thermonectus marumoratus, a highly effective visually guided predator that has evolved a collection of unusual visual mechanisms. Anatomically, these larvae have long tubular eyes, with multiple linear retinas that result in narrow visual fields. To increase their visual range, larvae routinely scan their surroundings with dorso-ventral head movements. The more proximal retina of the principal eyes (situated furthest from the lens) is UV sensitive, and may be specialized to recognize the polarization patterns of their prey. The more distal retina consists of multiple layers of green- sensitive photoreceptors, and could assist in assessing prey distance. In addition, these eyes have truly bifocal lenses, which likely provide separate sharp images to each of the two main retinas. Unlike more familiar bifocal lenses, the larval lenses exhibit an asymmetry that separates the two images, projected by the bifocal lens, in two dimensions: dorso-ventrally and in depth. This organization could possibly allow the larvae to see its prey with improved contrast, a mechanism that could potentially be exploited in optical engineering.
How might such a sophisticated and highly specialized eye have evolved? Stemmata are thought to have arisen from a compound eye- like ancestor, and a comparison to other diving beetles reveals the relatively recent origin of the enlarged tubular eyes. Studying the embryonic development of the T. marmoratus eyes, several structural similarities to the development of compound eye facets are apparent. Based on molecular genetic studies in Drosophila we know that key eye development genes have vertebrate orthologs, yet their roles in invertebrates other than Drosophila remain largely unknown. In a recent transcriptome analysis of T. marmoratus larvae we were able to find orthologs of several key genes (such as eyes absent, sine oculis, glass; prominin, crumbs, orthodenticle and prospero), the regulation of which might well play a role in the development of these extraordinary eyes. Investigating the roles of these genes in organisms such as T. marmoratus larval eye development could perhaps tell us, more generally, how the anatomical and functional diversity in visual systems might arise. While most Neuroethologists are familiar with applying Krogh’s principle to the study of physiology and function, perhaps the time is now ripe to use known developmental genetic mechanisms and apply them to the more extreme forms of the diversity of life.
Acknowledgements
This work was supported by the NSF (IOS0545978 and IOS1050754).
Keywords:
eye evolution,
Vision
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Plenary Address (including special lectures) (Note, these individuals have already been invited)
Topic:
Sensory: Vision
Citation:
Buschbeck
EK
(2012). The making of an eye: structural and functional diversity of stemmata..
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00044
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Received:
30 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Dr. Elke K Buschbeck, University of Cincinnati, Biological Sciences, Cincinnati, Ohio, 45221-0006, United States, elke.buschbeck@uc.edu