Volvox 2015: evolution

This is taking much longer than I ever expected; hopefully I can get through blogging about Volvox 2015 before registration opens for Volvox 2017!

The final session on day 1 (August 20) was chaired by Aurora Nedelcu from the University of New Brunswick. Dr. Nedelcu’s introduction emphasized some of the basic questions in evolutionary biology, aside from the origins of multicellularity and sex, on which volvocine research has provided insights: the evolution of morphological innovations, the relative importance of cis-regulatory changes vs. protein-coding changes, kin vs. group selection as competing explanations for the evolution of altruism, the evolution of soma and of indivisibility, the genetic basis of cellular differentiation, and the role of antagonistic pleiotropy (my hastily scribbled notes seem to say “antagonistic pleiotropy of olsl.” Is that supposed to be rls1? This is the cost of waiting too long to write. Maybe Aurora can clarify.).

I presented some new results from the Chlamydomonas experimental evolution project. in two separate experiments, we have observed the de novo evolution of simple multicellular structures from outcrossed starting populations. In the first experiment, simple multicellular structures evolved in response to selection for increased size by low-speed centrifugation. In the second experiment, colonial forms reminiscent of Pandorina or Volvulina evolved in response to predation by the ciliate Paramecium tetraurelia. The form of multicellularity observed differs substantially between experiments, suggesting that the particulars of the transition to multicellular life depend not only on the nature of the unicellular ancestor, but on the specific selective pressures driving the transition as well. We are currently working to understand the genetic changes underlying the transition to multicellular life in both experiments.

Maggie Boyd, an undergraduate in my lab group, showed her recent results measuring motility in Chlamydomonas isolates derived from the predation experiment. Starting with well-mixed cultures, she exposed them to directional light and measured the resulting changes in density. Sadly, despite their resemblance to small colonial volvocines, the colonies that evolved in this experiment appear unable to swim (or at least to swim toward a light). Phototaxis is important for photosynthesis, so the inability of these colonies to swim suggests a trade-off between viability (protection from predators) and reproduction, a trade-off we may want to test.

Erik Hanschen, from the University of Arizona, presented some results from the sequencing of the Gonium pectorale genome. Most of these results are unpublished, so I’m not going to say much about them here. If I’m being overly cautious, Erik is welcome to comment or guest post about these results. Erik’s previous work has shown that at least part of the genetic basis for somatic cell differentiation was likely present in the undifferentiated ancestors of Volvox (recall that the nominal genus Volvox is polyphyletic). The release of a third volvocine genome will be a welcome addition, substantially reducing uncertainty about which features are ancestral to the multicellular species (those shared between Gonium and Volvox) and which are derived in the V. carteri lineage.

The final talk in the Evolution session was by Professor Pauline Schaap FRSE from the University of Dundee, this year’s invited speaker. Prof. Schaap studies cellular slime molds including Dictyostelium, another well-established model system for understanding the evolution of multicellularity. Multicellular development in cellular slime molds (or social amoebae) is fundamentally different from that in the major multicellular radiations (red, Ulvophyte and Chlorophyte green, and brown algae; animals; plants; and fungi). Rather than clonal development from a single cell or propagule, cellular slime molds develop through aggregation, in which free-living cells come together to form a multicellular structure. Because of this I’ve been pretty skeptical in the past about the relevance of Dictyostelium to understanding the origins of multicellularity. I have to admit, though, that the list of aggregative developers Prof. Schaap showed in her introduction was much longer than I was aware of:

Organisms with aggregative multicellularity.

Fig. 1 from Du et al. 2015: Organisms with aggregative multicellularity.

The social amoebae share some of the strengths of the volvocine system, including species with and without cellular differentiation and an array of extant species that span a range of sizes and degrees of complexity. Prof. Schaap and colleagues have taken advantage of this diversity to reconstruct ancestral character states in a phylogenetic context (Romeralo et al 2013), concluding that some traits in the multicellular structures had multiple, independent origins. By combining phylogenetic comparative methods with developmental genetics, Prof. Schaap’s group investigated the evolutionary history of genes underlying multicellular development in social amoebae (Kawabe et al. 2015), many of which involve signals mediated by cyclic AMP. These signals appear to have been co-opted from an ancestral stress response:

In conclusion, we propose that the range and complexity of the cAMP-mediated signalling cascades that control almost all stages of multicellular development in modern Dictyostelia have been co-opted from an ancestral role for cAMP in stress-induced dormancy. The emergence of complex cell-cell communication from basic environmental sensing in Dictyostelia may prove to be a paradigm for the evolution of multicellularity in other eukaryote lineages. [Emphasis mine]

Co-option of genes involved in multicellular development from an ancestral role in environmental sensing…sounds familiar. Aurora Nedelcu and colleagues have proposed a similar scenario for the origin of regA, a crucial gene for determining somatic cell fate in Volvox carteri. Prof. Schaap may be right that co-option of genes involved in environmental sensing commonly play a role in the evolution of multicellularity.

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