Conservation and Radical Evolutionary Changes in Early Development

Lecture by Chris Cameron

BIOL 606 Session, University of Alberta, March 30, 1999.

Rapporteur: Tricia Abe

The lecturer, Chris Cameron, presented information to elucidate the controversies surrounding the construction of phylogenetic relationships based on larval and adult characters. An central issue is whether the evolutionary histories of larvae and adult forms may be considered independently from each other. Because larval and adult forms are subjected to different selection pressures, their evolutionary histories (some would argue) may be considered independently from each other. Alternatively, divergence in larval life-history traits may significantly influence the evolution of the adult stage. The degree of coupling between larval and adult forms, and the implications for their evolutionary trajectories, were explored in this lecture.

The first stages of early development are highly conserved for animals that develop from larvae. Arguments for the slow evolution of early developmental stages iclude: 1) the conservation of larval forms over long periods of time, seen in the fossil record (eg. Nauplius larvae) 2) conservation of cleavage patterns and fate maps 3) the mechanical difficulties of making changes in early development, since mutations early on in development may inhibit developmental progression in subsequent stages. The degree of influence that each change in a trait has on other traits was termed "generative entrenchment" by Wismatt and Shank (1988). Presumably, the convergent morphology of larval species is due to tightly conserved early development, and similar adaptations to particular life-history strategies.

However, in some cases, evolutionary modifications occur early in development. A transition from indirect developers (planktonic larvae) to direct developers (benthic larvae) has occurred in many times in echinoderrms. A mechanism to explain this transition was suggested by a study of early chordates (tunicates). Hybridizing a planktonic and a benthic tunicate species blocked the expression of one gene complex. Blockage of this gene caused significant morphological changes in the resulting hybrid (loss of an eyespot and tail). Thus, a transition from a planktonic to a benthic form may be faciliated by few genes.Given that the transition from planktonic to benthic forms has occurred in nature, this raises intriguing questions about the consequences of divergent larval life-history for the evolution of adult forms.

A diversity of larval forms is seen in echinoderm pluteus larvae, from obligate feeding larvae to obligate non-feeding larvae ("schmoos"). Chris presented adaptive explanations for the switch from indirect, feeding larvae to direct, non-feeding larvae. For example, direct development may have evolved in response to selective pressures from high predation in the plankton or from unreliable larval food sources. The consequences of divergent larval life-history characteristics, including dispersal capacity, were discussed.

Indirect developers would maintain genetic diversity by outbreeding, thus promoting species longevity and a low rate of speciation. Loss of larval feeding structures and reduced mobility in direct developers would minimize gene flow between populations, promoting inbreeding and a loss of genetic diversity. Thus, direct developers are expected to exhibit rapid rates of speciation and extinction. Furthermore, significant changes to the underlying developmental mechanisms, including faster rates of metamorphosis, occur with the evolution of direct developing larvae. This results in convergent larval forms. The Concentrated Changes Test ( Wray and Bely 1994) calculates the likelihood that such correlations in larval life history traits occur by chance.

Chris concluded his talk by emphasizing that the developmental mechanisms of sea urchins and ascidians can evolve quite rapidly, while evidence for very slow evolution of larval forms is seen in other groups. This complicates the process of using larval traits in phylogentic analysis, leaving the relationship between the evolution of larval form and species evolution open to speculation.


Discussion

Discussant: Grant McIntyre

Mark Wilson questioned the assumption that early development has been highly conserved based on evidence from the fossil record. If direct developers (schmoos) are not as likely to be represented in fossils, perhaps development hasn’t been conserved at all. Rich Palmer pointed out some of the selective advantages that would promote transitions from non-direct to direct lifestyles, in both directions.

Grant McIntyre wondered how limited Schmoo dispersal is, anyways. It is not clear how extensive dispersal is in direct developers. Sean Graham questioned the adaptive value of evolving an adult form, since the transition from larva to adult seems so complex, and wondered why the larvae don’t just stay in the plankton. Chris responded that the invertebrate larvae that forgo the transition to an adult life in the benthos can be seen in fish (although this observer detected a note of irony ).

After the break, Grant opened the discussion by addressing a confusing statement in the focal paper. It concerned an assertion by the authors that changes in life-history traits all occur in one direction (indirect to direct developers) and this is probably due to mutations, rather than selection. Shaky rationale was acknowledged by all. Rich gave an example using cave fish that supports the idea that random mutations are responsible for adaptive changes. Fish living in caves adapt to dark environments by not developing eyes. Hybrids between two populations of fish with equally reduced eyes develop eyes, supporting the idea that random loss of non-functional genes leads to reduced eyes. A new genetic background in hybrids corrects for this random loss and leads to the production of full eyes. However, it is still difficult to ascertain the relative influences of selection and random mutations.

Grant asked if anyone knew of examples of life-history traits being uncoupled between adults and larvae, since juvenile life-history traits like egg size, size at hatch, and development rate may have a significant effect on adult life-history. Discussion followed regarding the relative influence of larval life-history traits on adult survivorship, particulary for species with varying ratios of time spent as larva to time spent as adults. The focal paper suggested that evolution of larval and adult stages are independent of each other since there are no correlations in patterns of change for adult and larval characters. Sean noted that there would have to be strong divergent selectional differences between adult and larval stages in order to detect a difference.

There was some discussion on differential selection pressures for shmoos and planktonic forms. Location in the water column is probably more important than body form for determing the selection differentials.

Grant noted that two obscure models were raised in the last paragraphs of the focal paper. There was general confusion over what they were and why they were included at the end of the paper.