Antónia Monteiro and William Piel Proposal number: IOS-0818731 "The evolution of serial homology: a case study with butterfly eyespots."
The bodies of complex organisms have been found to reuse developmental structures, such as teeth, vertebra, and ribs, in order to build "serially" repeated parts or processes. This composite or modular design suggests that certain genetic programs are redeployed multiple times in different places during the developing embryo. It remains to be understood how such modules come to be redeployed over the course of evolutionary diversification. This project aims to explore the evolutionary and developmental origin of complex repeated structures by studying serially-repeated "eyespot" patterns in the wings of butterflies. The researchers will assemble a large phylogenetic tree of nymphalid butterfly species, photograph and code their wing patterns with respect to the presence or absence of spots or eyespots, and use comparative methods to infer how eyespots evolved. In addition, the evolution of the eyespot gene developmental "network" will be investigated by monitoring the expression of four candidate network genes in twenty species chosen on the basis of phylogenetic distribution of wing pattern evolution.
This project has considerable educational Broader Impacts over a wide range of educational levels, supporting a graduate student, postdoc, and several undergraduate work-study students. Moreover, several New Haven high school student researchers will be involved through "Evolutions", the Peabody Museum's outreach program to city youth. This project will also generate a publicly accessible high-resolution image database of butterfly species (ca. 8,000 images from 1,000 nymphalid species), a large phylogeny of nymphalids, and the first database for gene expression images in the Lepidoptera. In addition, interactive web-accessible animations will be produced and incorporated into the exhibits of the Peabody Museum. This software will illustrate the diversity of nymphalid wing patterns and will animate the evolution of eyespot changes on phylogeny. This exhibit will introduce the public to the diversity and evolution of wing patterns, the evolution of individuation of repeated modules, and the evolution of morphological novelties such as eyespots.
During the period of the award we investigated how traits that are often found repeated in organisms, such as limbs, segments, teeth, vertebrae, and wings, originate and evolve in number. Currently we do not understand how most of these traits became repeated in a body. For instance, if you think of insect limbs, we currently do not know whether these limbs appeared simultaneously across all body segments, or whether they originated in one part of the body and then were recruited and deployed in novel body regions. There are two hypotheses for the origin of repeated and individuated traits in bodies: bodies can start out by evolving one or more units at particular locations, and then repeating these units in novel positions along the body. Or, alternatively, they can evolve multiple units that initially function as a single "trait" that then become individuated over the course of evolution, allowing each unit to evolve independently of the others. Our study system was nymphalid butterfly eyespots. There is much variation in the number of eyespots that nymphalid butterflies display on their wings today but, as with many other repeated traits in bodies, we did not know how these repeated traits originated and evolved. We photographed and scored eyespot presence and absence on the wings of 400 species of nymphalid butterflies belonging to different genera. By reconstructing ancestral traits we discovered that nymphalid eyespots originated once, in a narrow window, approximately 95 to 85 million years ago. In addition, we discovered that eyespots originate as a cluster of 4 to 5 units on the ventral surface of the hindwing. Eyespots later appeared in the anterior wing and on the dorsal surface of both wings. Our data suggests, thus, that these repeated traits originated as a small cluster of units on one part of the body that was later moved to novel parts of the body over evolutionary time. This mode of evolution is similar to the current hypothesis for the origin and evolution of the paired vertebrate limb. The fossil record suggests that paired limbs originated first in the anterior part of the body (as pectoral fins) and were later deployed in a more posterior body regions, giving rise to pelvic fins. In addition, we investigated how the gene regulatory network that is associated with the development of eyespots originated and evolved. We compared the expression of 5 genes across 22 species of nymphalids with eyespots to try and understand how such a network builds up over evolutionary time. Our broad comparative approach revealed a novel, previously undocumented pattern of regulatory network evolution. Four of the five genes were reconstructed as having gained eyespot-specific expression concurrently with eyespot origins. Furthermore, eyespot-specific expression for many of these genes was lost in multiple lineages without loss of eyespots. This indicates a rapid, potentially simultaneous appearance of multiple genes expressed in the eyespots followed by considerable network simplification. This suggests, for the first time, that novel traits may originate from the co-option of gene networks that are overly complex for their novel role in development, and which are later streamlined of unessential genes over the course of evolution.
