-
[
Bioessays,
2008]
Homology is the similarity between organisms due to common ancestry. Introduced by Richard Owen in 1843 in a paper entitled "Lectures on comparative anatomy and physiology of the invertebrate animals", the concept of homology predates Darwin's "Origin of Species" and has been very influential throughout the history of evolutionary biology. Although homology is the central concept of all comparative biology and provides a logical basis for it, the definition of the term and the criteria of its application remain controversial. Here, I will discuss homology in the context of the hierarchy of biological organization. I will provide insights gained from an exemplary case study in evolutionary developmental biology that indicates the uncoupling of homology at different levels of biological organization. I argue that continuity and hierarchy are separate but equally important issues of homology.
-
[
Evolution & Development,
1999]
Developmental constraints have been defined as biases on the production of variant phenotypes or limitations on phenotypic variability caused by the structure, character, composition, or dynamics of developmental systems". The term has widely been discussed, but is far from being generally accepted. One reason might be that is has so far not been possible to test experimentally the concept of restraint. Experimental tests are difficult because the hypothesis is supported only by the failure or the limitation of seeing phenotypic variation, which can always hypothetically be explained by stabilizing selection. This article suggests an approach to study the potential molecular basis for developmental constraints.
-
[
Bioessays,
1997]
To understand how morphological characters change during evolution, we need insight into the evolution of developmental processes. Comparative developmental approaches that make use of our fundamental understanding of development in certain model organisms have been initiated for different animal systems and flowering plants. Nematodes provide a useful experimental system with which to investigate the genetic and molecular alterations underlying evolutionary changes of cell fate specification in development, by comparing different species to the genetic model system Caenorhabditis elegans. In this review, I will first discuss the different types of evolutionary alterations seen at the cellular level by focusing mainly on the analysis of vulva development in different species. The observed alterations involve changes in cell lineage, cell migration and cell death, as well as induction and cell competence. I then describe a genetic approach in the nematode Pristionchus pacificus that might identify those genetic and molecular processes that cause evolutionary changes of cell fate specification.
-
[
Dev Suppl,
1994]
The invariant development of free-living nematodes combined with the extensive knowledge of Caenorhabditis elegans developmental biology provides an experimental system for an analysis of the evolution of developmental mechanisms. We have collected a number of new nematode species from soil samples. Most are easily cultured and their development can be analyzed at the level of individual cells using techniques standard to Caenorhabditis. So far, we have focused on differences in the development of the vulva among species of the families Rhabditidae and Panagrolaimidae. Preceding vulval development, twelve Pn cells migrate into the ventral cord and divide to produce posterior daughters [Pn.p cells] whose fates vary in a position specific manner [from P1.p anterior to P12.p posterior]. In C. elegans hermaphrodites, P(3-8).p are tripotent and from an equivalence group. These cells can express either of two vulval fates (1 or 2) in response to a signal from the anchor cell of the somatic gonad, or a non-vulval fate (3), resulting in a 3-3-2-1-2-3 pattern of cell fates. Evolutionary differences in vulval development include the number of cells in the vulval equivalence group, the number of 1 cells, the number of progeny generated by each vulval precursor cell, and the position of VPCs before morphogenesis. Examples of three Rhabditidae genera have a posterior vulva in the position of P9-P11 ectoblasts. In Cruznema tripartitum, P(5-7).p form the vulva as in Caenorhabditis, but they migrate posteriorly before dividing. Induction occurs after the gonad grows posteriorly to the position of P(5-7).p cells. In two other species, Mesorhabditis sp. PS 1179 and Teratorhabditis palmarum, we have found changes in induction and competence with respect to their presumably more C. elegans-like ancestor. In Mesorhabditis, P(5-7).p form the vulva after migrating to a posterior position. However, the gonad is not required to specify the pattern of cell fates 3-2-1-2-3. Moreover, the Pn.p cells are not equivalent in their potentials to form the vulva. A regulatory constraint in this family thus forces the same set of precursors to generate the vulva, rather than more appropriately positioned Pn.p cells.
-
[
Worm Breeder's Gazette,
1993]
We have started an evolutionary analysis of vulva formation in different nematode species. One obvious morphological difference is between species with a vulva in the central versus posterior body region. In the family Rhabditidae species with a posterior vulva are known in the genera Mesorhabditis, Teratorhabditis and Cruznema, which are thought to belong to three different evolutionary lines. In the strains Rh. (Mesorhabditis) spec., (PS 1179, Sternberg Lab-collection) and Rh. (Cruznema) tripartita the vulva forms at 80% body length, whereas in Rh. (Teratorhabditis) palmarum the vulva forms at 95% body length, just anterior to the anus. Which P-ectoblasts form the vulva? Taking Panagrellus with P(5-8).p making the vulva as an example, it seemed most likely that the more posterior P-cells generate vulva tissues in this species. But cell lineage analysis revealed that in all three species the vulva is formed by the central Pn.p- cells (P4.p-P8.p and P3 .p-P8.prespectively) after migration to the posterior body region. In addition to the posterior vulva, all these species have a monodelphic gonad. The gonad primordium is located in the central position like in C. elegans, and grows towards the posterior during the L2 and L3 .There is thus no contact between the VPCs and the gonad in the early L3 ,when vulva induction occurs in C. elegans. How does vulva induction takes place in these species? In Cruznema the VPCs don't divide until the AC contacts P6 .pin the late L3 .Indeed, ablation of Z(1,4) in the L1 or the AC in the early L3 prevents vulva formation and give rise to 3 lineages in all the VPCs. We therefore conclude that vulva formation is gonad dependent in Cruznema and that the signaling event takes place in the late L3 .This difference in the timing of vulva induction with respect to C. elegans is a clear case of heterochrony. In Mesorhabditis and Teratorhabditis the VPCs undergo at least two rounds of cell divisions before the gonad contacts the forming vulva. Ablations of Z(1,4) in the L1 results in normal vulva formation. Thus vulva development is gonad-independent in these species. To understand VPC pattern formation in this species we did various 1, 2 or 3 - VPC isolation experiments. It is very striking that a cell of the pair P 5,6 L/R is always 1 over any other cell, independent of whether it is in the anterior or posterior position. Furthermore, in the "1-cell isolation" only P5 .pand P6 .pare able to express the correct 1 fate. In contrast, an isolated P7 .por P8 .pshow a hybrid lineage with an incorrect AC contact and abnormal invagination in later stages. P4 .pexpressed only the 3 fate as an isolated cell. The result of different "3 cell isolations" also suggests that the VPCs are not equivalent to each other; P5 .pwas always 1 over P7 .p.This was also seen after ablation of just P6 .p:P5 .pwas 1 in all 21 animals, an experiment that gives different results in C. elegans. Our current working model contains several steps in the vulva-formation process: 1.) An yet unidentified inductive signal, probably from the posterior body region and probably redundant. 2.) A prebias of the VPCs making P5 .pand P6 .pmore likely to adopt the 1 cell fate. In the intact situation P6 .padopts the 1 fate, probably because it gets the inductive signal earlier or in a higher dose. 3.) Intercellular signaling events specify the final cell fate pattern in the equivalence group, like in C. elegans.
-
[
Curr Opin Genet Dev,
2000]
Multiple evolutionary variations occur in the cellular and genetic programming of nematode development. Many changes involve alterations of inductive interactions. Surprisingly, inductive processes vary during evolution, irrespective of changes in the final cell lineages and morphological structures. Genetic studies in some nematodes also shed light on the underlying mechanisms of evolutionary change.
-
[
Nat Rev Genet,
2009]
There has been a recent trend in evolutionary developmental biology (evo-devo) towards using increasing numbers of model species. I argue that, to understand phenotypic change and novelty, researchers who investigate evo-devo in animals should choose a limited number of model organisms in which to develop a sophisticated methodological tool kit for functional investigations. Furthermore, a synthesis of evo-devo with population genetics and evolutionary ecology is needed to meet future challenges.
-
[
Dev Biol,
1996]
Nematodes provide a useful experimental system with which to investigate the evolution of development at the cellular, genetic, and molecular levels. Building on an understanding of vulval development in Caenorhabditis elegans, analysis of vulval development has been extended to a number of other species in three families of the Nematode phylum. Changes have occurred in most aspects of vulval development: alteration in the number of cells competent to participate in vulval development by changes in apoptosis; changes in the relative contributions of position-dependent predisposition toward particular fates (prepattern), inductive signaling and lateral signaling; and in the specific lineages generated by vulval precursor cells. Genetic analysis of one species in which only three vulval precursor cells are present identified a mutation that increases the number of vulva precursor cells toward that found in C. elegans.
-
[
Invertebrate Reproduction and Development,
1999]
Over the last few years vulva development in nematodes has been used as a model system to study the evolution of developmental processes by carrying out cell lineage and cell ablation studies in various nematodes. Furthermore, a genetic and molecular analysis of vulva development has been initiated in Pristionchus pacificus. Evolutionary interpretation of these comparative developmental studies requires a phylogenetic understanding of nematodes. Recently, a molecular phylogeny for the phylum Nematoda has been published. Here, we place the comparative data of vulva development onto this phylogeny of nematodes to infer the direction of evolutionary change.
-
[
Curr Biol,
2000]
Recent studies have introduced Oscheius sp. CEW1 as a third nematode species accessible to genetic analysis, joining the better known Caenorhabditis elegans and Pristionchus pacificus. A group of vulva-defective mutants in Oscheius has been identified, with defects not seen in C. elegans.