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Despite the widespread conservation of a basic pluteus larval form in most sea urchins, there are several types where selection has favored those lacking a pluteus larval stage. Among these species of direct-developers is one of special interest; Heliocidaris erythrogramma, which shows stunning homology in its adult life form to its larval-bearing, (indirect developing) sister, H. tuberculata. Although the two diverged only approximately ten million years ago, they each display dramatically different morphogenesis from cleavage to juvenile forms (Raff, 1996). In the past few years of research on this phenomenon in sea urchins, several questions have been posed and answered with varying absolution. Biologists have proposed several ecological suppositions to explain the occurrence of direct-development and have illustratred many clearly advantageous aspects in terms of evolution, but perhaps the least defined in the general arena of speculation for the change in development patterns, lies in the determination of the specific molecular mechanisms that dictate this morphological difference. Having homologous juvenile and adult forms, however divergent cleavage patterns and molecular specification of territories, H. erythrogramma and H. tuberculata have proven to be ideal species for the examination and mapping of the selective pressures and molecular mechanisms responsible for the evolution of such a novel mode of development.
By examining sea urchin phylogenies, one can estimate with little doubt that indirect development via a free-swimming larva is the ancestral form of sea urchins. Coincidentally, it is also important to note that, in these phylogenies, direct-developing species seem to be "embedded" amongst species that have conserved the larval pluteus stage (Raff, 1996). This would imply that the change to direct development has evolved independently in several lineages (Emlet and Hoegh-Guldberg, 1996) and thus merits gradation of a viable means of progressive adaptation. However, before attempting to delineate the process of selection that has occurred across the class, identification of the ecological advantages and disadvantages of direct- and indirect development must be made.
The larval morphogenetic stage of H. tuberculata occurs as the 32-cell stage embryo transforms to a pluteus larva which is capable of swimming and feeding and which disperses as far away from the maternal urchin as its 4 weeks of development will allow (Gilbert, 1994). Often, the larva will settle in an environment that is favorable in terms of food availability, protection and temperature, to settle and begin adult morphogenesis. As a commonly observed phenomenon among many larval echinoderms, the high risk of predation by larger filter feeders is outweighed by the advantages of wide dispersal allowing for increased genetic variation in populations, and countered by the effects of higher fecundity in indirect developers. Since direct-developing sea-urchins are rare in the Northern Hemisphere, but common in extremely deep or shallow waters of Antarctica and Australia (Emlet and Hoegh-Guldberg, 1996), one may speculate that the emergence of H. erythrogramma was due to the adaptation of a new lineage in response to extremes of depth and temperature resulting in new niches. It is also possible that these changed conditions created pressures where feeding was concerned. To illustrate this possibility, it has been observed that when experimentally starved, Clypeaster rosaceous can facultatively switch to direct-development and, under normal conditions, develop a feeding larva (Emlet and Hoegh-Guldberg, 1996). Facultative developers such as this species can be seen as intermediate stages in the evolution of types such as H. erythrogramma.
Where means of evolution is concerned, it has been observed that H. tuberculata produces eggs around 100Mm in diameter, whereas the eggs of H. erythrogramma range from 300 to 2,000 Mm in diameter (Henry et al.). The much larger egg size in H. erythrogramma can be attributed to the more nutritive yolk required to give rise to the juvenile urchin so early in development, compared to H. tuberculata. With increased egg size having been selected for in a population, ensuing developmental changes would be diverse and relatively easily accomplished, given the right conditions (Laegdsgaard et al, 1991). In that the loss of the larval form in ancestral indirect developers a common convergent trend, one can deduce that the molecular mechanisms by which the changes in development and morphogenesis occur must be relatively simple, especially where body structure as in H. tuberculata and H. erythrogramma shows such striking resemblance.
Research using fluorescent tracer dyes to determine cell fate maps of both H. tuberculata and H. erythrogramma shows that radical modifications of morphogenesis in H. erythrogramma did not occur by a single gene mutation, but rather, through a series of relatively simple mechanisms of change (Raff, 1996). Perhaps the most obvious and instrumental morphological differences between the two species is that of cleavage pattern and subsequent cell determination of fates. Cleavage patterns of the two species differ in that the early celles of H. erythrogramma are of the same size, whereas in H. tuberculata, unequal cleavages result from cells of different sizes found at the 16 and 32 cell stages of the embryo (Raff, 1996). It can be seen in fate maps, that in both species, extodermal development determines the same cells, but in different locations, and distributed differently among cells. For instance, in H. erythrogramma, ectodermal cells are shown mostly dorsally, as opposed to the bilateral symmetry of ectodermal cells in H. tuberculata (Emlet, 1995). Furthermore, the internalization of a large portion of the ectoderm, including many skeletogenic and coelemic cells provide grounds for the evolution of a new method of gastrulation (Raff, 1996). Another important factor in identifying the types of change that evolved in embryonic development is the location of the ciliated band in the two types of urchins. Further experiments using cell tracing methods have revealed that cell-cell communication, and not simply cell-lineage, as previously thought, was responsible for the location of the cilliary band that is present in both H. erythrogramma and H. tuberculata (Emlet, 1995). Considering the rearrangement of cells in the 32-cell state of the H. erythrogramma embryo, one can conclude that the movement of the signal cells resulted in the subsequent relocation of the cilliary band.
With regards to morphogenesis, the process shows distinction in H. erythrogramma from cleavage onward (Raff, 1996). In H. erythrogramma, a wrinkled blastula is formed, instead of the smooth blastula characteristic of H. tuberculata. The gastrulation that follows also has extreme differences. The changes occur in the second stage of gastrulation, where archenteron elongation takes place, and unlike H. tuberculata, which simply rearranges component cells, H. erythrogramma involutes a sheet of cells from the ventral side of the embryo. Although the precursor of this mechanism is not known, biologists have discovered that this adaptive way of elongating has evolved to provide raw material for the further accelerated adult morphogenesis (Henry et al, 1990).
Finally, in observing the drastic differences in morphologies of the two species of sea urchin discussion, it would seem evident that their evolutionary divergence was due to large variance in gene expression. While this is highly probable, in that there is sufficient evidence to show that new genes have evolved in H. erythrogramma, it would perhaps be even more appropriate to relate the evolution of such a modified development upon the timing of gene expression before reaching the embryonic stage (Raff, 1996). An examination of a specific kind of gene, the actin gene family has provided much information on the effects of timing in gene expression as mechanisms of modification. Actin genes are known to play important roles in cells during development and are thus ideal for the examination of gene expression differences between different embryos. According to research, cytoplasmic actin is expressed in the aboral ectoderm cells of indirect developers, but has been discovered as an unexpressed pseudogene in H. erythrogramma (Kissinger and Raff, 1998). It is commonly upheld that this pseudogene could be an initiator of a new developmental modes in an ancestral species, although the reasons are not fully understood (Raff, 1996).
Given their drastically different patterns of development and morphogenesis, an observer of the egg to embryo to juvenile forms of the sea urchins, Heliocidaris tuberculata and Heliocidaris erythrogramma would likely classify each into seperate genuses based on dissimilarities of morphology. One would probably not consider that they were, indeed of the same genus and had essentially identical juvenile and adult morphologies. Nonetheless, a great deal about the mechanisms of such changes in development must be understood as well as the various possible environmental factors must be understood before an attempt to classify such organisms according to ontogeny. As one may conclude, it is rarely a simple, random fluctuation in a single gene that brings about such profound modifications while at the same time, preserving many ancestral, unifying traits, as exemplified in H. tuberculata and H. erythrogramma. Moreover, it is a sequential process of subtle modifications in cell lineage, cleavage, morphogenesis and gene expression, combined with favorable conditions for selection that govern the evolutionary fates of divergent organisms, such as H. tuberculata and H. erythrogramma.
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