Evolution of regeneration and fission in annelids: insights from engrailed- and orthodenticle-class gene expression.

The recent explosion of information on the role of regulatory genes in embryogenesis provides an excellent opportunity to study how these genes participate in post-embryonic developmental processes. We present a detailed comparison of regulatory gene expression during regeneration and asexual reproduction (by fission) in the segmented worm Pristina leidyi (Annelida: Oligochaeta). We isolated three genes from Pristina, one homolog of engrailed and two homologs of orthodenticle, and characterized their expression in different developmental contexts. In situ hybridization studies on worms undergoing normal growth, regeneration and fission demonstrate that in all three processes, Pl-en is expressed primarily in the developing nervous system, and Pl-Otx1 and Pl-Otx2 are expressed primarily in the anterior body wall, foregut and developing nervous system. Our data reveal extensive similarities between expression during regeneration and fission, consistent with the idea that similar developmental processes underlie these two types of development. Thus, we argue that in these annelids fission may have evolved by recruitment of regenerative processes. Furthermore, by comparing our data to existing data from leech embryos, we find evidence that embryonic processes are re-deployed during regeneration and fission.

Stochastic WNT signaling between nonequivalent cells regulates adhesion but not fate in the two-cell leech embryo.

BACKGROUND: In the leech Helobdella robusta, an annelid worm, the early pattern of cell divisions is stereotyped. The unequal first cleavage yields cells AB and CD, which differ in size, cytoplasmic inheritance, normal fate, and developmental potential. RESULTS: Here we report a dynamic and transcription-independent pattern of WNT signaling in the two-cell stage of H. robusta. Surprisingly, HRO-WNT-A is first expressed in a stochastic manner, such that either AB or CD secretes the protein in each embryo. This stochastic phase is followed by a deterministic phase during which first AB, then CD expresses HRO-WNT-A. When contact between the cells is reduced or eliminated, both AB and CD express HRO-WNT-A simultaneously. Finally, bathing embryos in anti-HRO-WNT-A antibody during first cleavage reduces the adhesion between cells AB and CD. CONCLUSIONS: Our findings show that the stochastic phase of HRO-WNT-A signaling in the two-cell stage of Helobdella is negatively regulated by cell-cell contact and that this early signaling affects cell adhesion without affecting cell fate. We speculate that the primordial function of wnt class genes may have been to regulate cell-cell adhesion and that the nuclear signaling components of the wnt pathway arose later in association with the evolution of diverse cell types.

The evolution of echinoderm development is driven by several distinct factors.

We analyzed a comparative data base of gene expression, cell fate specification, and morphogenetic movements from several echinoderms to determine why developmental processes do and do not evolve. Mapping this comparative data onto explicit phylogenetic frameworks revealed three distinct evolutionary patterns. First, some evolutionary differences in development correlate well with larval ecology but not with adult morphology. These associations are probably not coincidental because similar developmental changes accompany similar ecological transformations on separate occasions. This suggests that larval ecology has been a potent influence on the evolution of early development in echinoderms. Second, a few changes in early development correlate with transformations in adult morphology. Because most such changes have occurred only once, however, it is difficult to distinguish chance associations from causal relationships. And third, some changes in development have no apparent phenotypic consequences and do not correlate with obvious features of either life history or morphology. This suggests that some evolutionary changes in development may evolve in a neutral or nearly neutral mode. Importantly, these hypotheses make specific predictions that can be tested with further comparative data and by experimental manipulations. Together, our phylogenetic analyses of comparative data suggest that at least three distinct evolutionary mechanisms have shaped early development in echinoderms.

Response of invertebrates to lotic disturbance: a test of the hyporheic refuge hypothesis.

Recovery following hydrological disturbances is usually rapid for lotic invertebrates. Stream ecologists have assumed that recovery is facilitated by behavioral migrations during floods down into the hyporheic zone (the interstitial spaces of a streambed) to seek temporary refuge from possible erosion (the "hyporheic refuge hypothesis"). We provide the first explicit test of this hypothesis by evaluating three predictions of the hypothesis. We coupled field observations of the response of meiofaunal invertebrates to floods with field and flume experiments. The study site was a sandy-bottom stream in northern Virginia. Prediction 1, that loss of fauna from a streambed during floods should be minimal as long as the depth of scour in the streambed is less than the depth of the hyporheic zone, was not supported for any taxon. For two floods which varied considerably in magnitude, 50-90% of the fauna was lost from the bed despite the fact that the depth of scour (10-30 cm) was significantly less than the total depth of the hyporheic zone (50 cm). Prediction 2, that fauna should move deeper into the bed at higher flows, was supported by field observations during only one of two floods and then only for rotifers. In flume experiments that tested for finer scale behavioral movements, significant vertical migrations were found for copepods and chironomids which moved 1.5-3.5 cm downward as mean velocity (3 cm off bottom) was increased from 5-23 cm/s. Movements down by rotifers were not found in the flume experiments. Prediction 3, that the hyporheic zone is the most important source of colonists to defaunated areas, was supported in part by field experiments. The hyporheic route was not the primary route for any taxon but it was as important for the rotifers and copepods as water column or streambed surface routes. We conclude that, even though smallscale (cm's) migrations into the streambed in response to increased flow may be observed for some taxa and the hyporheic zone may serve as a partial source of colonists following disturbances, movements down are not adequate in preventing significant losses of meiofauna during floods.