November 6, 2019
12:00 LSB 2320
National Museum of Natural History, Smithsonian Institution
"How development influences morphological diversity"
This project combines physiological, morphological, developmental, and genomic techniques to understand how selection pressures (extrinsic factors) and developmental bias (intrinsic factors) produced surprising convergent middle ear loss. Most tetrapods, including frogs and toads (anurans), have evolved tympanic middle ears that amplify airborne sound. Anurans are known to locate and attract mates via acoustic communication, yet many anuran species lack tympanic middle ears. We found middle ear loss is common among anurans (at least 38 losses) and we find very little evidence that extrinsic factors related to environmental selection pressures can explain convergent tympanic middle ear loss. We further compared hearing among eared and earless species using auditory brainstem recordings (ABRs) and found negative hearing consequences associated with ear loss at high frequencies but similarity in hearing between eared and earless species below 1 kHz. Using microCT, we compared skull morphology between eared and earless species to reveal ear loss did not result from pleiotropic trade-offs with developmentally linked skull structures. Larger cell and genome sizes as well as prolonged middle ear development of earless species, point towards shifts in development rate (heterochrony) influencing middle ear lability in anurans. Using phylogenomic data for 55 species (36 eared, 19 earless) within the earless frog system, we found earless lineages exhibit relaxed purifying selection in putative middle ear coding sequences and we are now deciphering how demography, drift, and selection affect genomic evolution in earless lineages. We conclude that a combination of relaxed selection on the middle ear, lack of integration with other skull features, and changes in development rate have contributed to convergent loss of middle ear structures. Earless anurans provide a great case study for how intrinsic factors (developmental bias) can shape macroevolutionary patterns.
Host: Karen Sears