[Since I had to fly away early this morning and missed all these talks, I had to rely on regular commenter DanioPhD to fill in the gaps … so here’s her summary:]
This morning’s final series of talks each focused on a different phylum, but the unifying theme was one of bridging the processes of microevolution and macroevolution. The first talk after breakfast (and a long night of Scotch-drinkin’ and story-swappin’ prior to that) was Bernie Degnan of the University of Queensland. He summarized his work on Amphimedon queenslandica, a sponge species developed as a model of a representative primitive metazoan. Sponges diverged from the metazoan lineage ca. 700 MYA and possess the most minimalist metazoan body plan–no nervous system, muscles, nor any discernible tissues in the adult body architecture. Their embryos, however, feature robust anterioposterior patterning, distinct cell types organized into tissues, and cell morphogenesis typical of more complex metazoans. These embryonic characteristics are achieved by a regulatory network of genes, which, while inactive in the adult sponge, strongly support the presence of similar molecules in the ancestral metazoan genome. A few million years after the divergence of porifera, metazoans were able to co-opt these molecular toolkits to build the diverse, molecularly and morphologically distinct tissues common to all bilaterians. PZ has previously written up one such sponge tale here describing the molecular precursors to a nervous system in the sponge genome. Precursors to pretty much every other developmental ‘big gun’, e.g, Hox genes, Pax genes, Wnts, Hedgehog, etc. are also present as a basic prototype, in the Amphimedon genome.
Next up at the podium was Michael Wade, of Indiana University. He presented a model for the evolution of maternal effect genes and made predictions based on algebraic expression of the rates of maternal effect alleles within and between populations. This was a bit outside my area/interests, but for more specifics, please see (this link to the relevant paper abstract). The take-home message was that the maternal genotype has critical input into embryonic development and survival, irrespective of the zygotic genotype. The mode of transmission and expression of maternal effect genes differ from zygotic genes, in that all offspring will be influenced by the maternal genotype, but only female offspring will activate these genes from their own genome. This gives natural selection different ‘access’ to these genes as compared to the garden variety zygotic gene pool, thus positive selection (selecting for advantageous mutations) and purifying selection (purging deleterious mutations) will work at different rates on maternal effect genes as compared to zygotic genes.
Bill Cresko of the University of Oregon presented his work on the evolution of bone morphogenesis in sticklebacks. This was another example of a microevolution study that may have applications useful for understanding macroevolution. Briefly, there are oceanic–andromatous, actually, as they migrate to fresh water for spawning–populations of sticklebacks that have existed in roughly their present form for over 10 million years. They feature some wicked bony spikes around their pelvic region and lateral dermal bones, both of which presumably confer some protection against predators. There are several fresh water species as well, young (15,000 years or less) populations descendent from the migratory oceanic stock who were trapped in fresh water by receding glaciers. The key feature of these new stickleback populations is their rapid (in evolutionary time) and repeated loss of both the pelvic spikes and the body armor. It happens in isolated freshwater stickle populations all over the world, over and over again. Bill’s group has been intercrossing the oceanic and freshwater stocks to identify the genes responsible for these changes. Mapping has revealed that the genetic regulators of pelvic spines are independent of those that regulate the lateral plates. From the gene clusters implicated in the latter, several interesting candidates with roles in mineralization, osteoblast function, and Calcium ion processing have been identified. Intriguingly, it’s not clear at all that selection is acting on the bony plate factors at all. It’s just as likely that some other linked factor is the real target of this rapid and repeated selection, and the bony plate genes on the same chromosome are just along for the ride.
Kevin Peterson, a paleontologist from Dartmouth College was the final speaker. He gave a broad and engaging summary of recent work on the evolution of microRNAs. microRNAs (miRs) are short (22-mer) RNA sequences which, after several steps of post-transcriptional modification to their shape and configuration, have precise, cell-specific effects on mRNA, either by degrading it altogether or inhibiting translation. Described as the ‘dark matter’ of the genome, these tiny, unassuming molecules were largely overlooked until recent investigations revealed their hitherto unknown importance in gene regulation. Peterson’s angle was to apply the known data on miRs across the phyla and take a novel look at the macroevolution of these factors. For years, evolutionary biologists have sought to explain the explosions of diversity and complexity seen in the vertebrate clade.
Morphological change plotted against time reveals that the biggest degree of change happened early in our lineage, and subsequent increases in complex phenotypes have been relatively miniscule. When the molecular era revealed the genome duplication phenomenon, many pinned their hopes on this as the explanation for complexity, but this rationale fails under further scrutiny. Similarly, as we learned from the Coyne-Wray duel on Friday night, Cis-regulatory elements were enthusiastically embraced as a key factor behind evolutionary complexity in vertebrates, but again, this explanation alone cannot account for many of the observed changes. Peterson and others have shown that miR gene families are present throughout all phyla, and have continued to evolve throughout evolution. There is a very low rate of secondary loss of miRs, once they appear, and sequence divergence is restricted due to the required nucleotide specificity of the recognition sequences which anneal to the target transcripts. These characteristics lend themselves to the creation of a robust phylogeny, which, in addition to clarifying some controversial assignments between morphological and molecular phylogenies, shows an acquisition of new miR families at a rate which largely overlaps that of the morphological changes noted in vertebrate evolution. Thus, the fine-tuning of cell-specific gene regulation conferred by the evolution of novel miRs may be an important mechanism in generating complexity.
[So now everyone has to tell DanioPhD to start her own blog!]