Abstract. One of the oldest problems in evolutionary biology remains largely unsolved. Which mutations generate evolutionarily relevant phenotypic variation? What kinds of molecular changes do they entail? What are the phenotypic magnitudes, frequencies of origin, and pleiotropic effects of such mutations? How is the genome constructed to allow the observed abundance of phenotypic diversity? Historically, the neo-Darwinian synthesizers stressed the predominance of micromutations in evolution, whereas others noted the similarities between some dramatic mutations and evolutionary transitions to argue for macromutationism. Arguments on both sides have been biased by misconceptions of the developmental effects of mutations. For example, the traditional view that mutations of important developmental genes always have large pleiotropic effects can now be seen to be a conclusion drawn from observations of a small class of mutations with dramatic effects. It is possible that some mutations, for example, those in cis-regulatory DNA, have few or no pleiotropic effects and may be the predominant source of morphological evolution. In contrast, mutations causing dramatic phenotypic effects, although superficially similar to hypothesized evolutionary transitions, are unlikely to fairly represent the true path of evolution. Recent developmental studies of gene function provide a new way of conceptualizing and studying variation that contrasts with the traditional genetic view that was incorporated into neoDarwinian theory and population genetics. This new approach in developmental biology is as important for microevolutionary studies as the actual results from recent evolutionary developmental studies. In particular, this approach will assist in the task of identifying the specific mutations generating phenotypic variation and elucidating how they alter gene function. These data will provide the current missing link between molecular and phenotypic variation in natural populations.
11The neural basis for behavioural evolution is poorly understood. Functional comparisons 12 of homologous neurons may reveal how neural circuitry contributes to behavioural 13 evolution, but homologous neurons cannot be identified and manipulated in most taxa.14 Here, we compare the function of homologous courtship song neurons by exporting 15 neurogenetic reagents that label identified neurons in Drosophila melanogaster to D. 16 yakuba. We found a conserved role for a cluster of brain neurons that establish a 17 persistent courtship state. In contrast, a descending neuron with conserved 18 electrophysiological properties drives different song types in each species. Our results 19suggest that song evolved, in part, due to changes in the neural circuitry downstream of 20 this descending neuron. This experimental approach can be generalized to other neural 21 circuits and therefore provides an experimental framework for studying how the nervous 22 system has evolved to generate behavioural diversity. 24peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/238147 doi: bioRxiv preprint first posted online Dec. 21, 2017; 2 however, do not sing sine song, but they produce two distinct modes of pulse song: thud 54 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission. Main textThe copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/238147 doi: bioRxiv preprint first posted online Dec. 21, 2017; 3 song and clack song 11 . Thud song is generated by unilateral wing vibration; while clack 55 song is generated when males vibrate both wings behind them (Fig. 1a) moving faster than when males sing pulse song (Fig. 1e, f). Additionally, males sing 68 pulse song mostly when they are located directly behind females, whilst they sing clack 69 song across a wide range of distances and positions relative to females (Fig. 1g) 11 . 70Consistent with these observations, removing motion signals by providing males with a 71 motionless decapitated female eliminated clack song but not pulse song (Fig. 1h, i shown to reduce wing extension during courtship 15 . We found that pIP10 inhibition in D.144 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/238147 doi: bioRxiv preprint first posted online Dec. 21, 2017; 6 melanogaster using our new split-GAL4 line caused almost complete elimination of 145 pulse song and a small reduction in sine song produced during normal courtship (Fig. 146 2a). In contrast, pIP10 inhibition in D. yakuba eliminated clack song consistently across 147 different split-GAL4 drivers and neuronal inhibitors (Fig. 2b and Extended Data Fig. 5a-148 c). In addition, in some treatments, pIP10 inhibition resulted in a quantitative reduction 149 of pulse song (Extended Data Fig. 5a, c). Th...
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