Biomedical subjects

J A Bolker

Publications and source records attributed to J A Bolker.

5 recordsLinked to original sources

PubMed2001

Homologies of process and modular elements of embryonic construction.

There are several signal transduction pathways that integrate embryonic development. We find that both within species and between species, these pathways constitute homologous modules. The processes, themselves, can be considered homologous, just as structures can be considered homologous. Just like vertebrate limbs, these pathways are composed of homologous parts (in this case, the proteins of the pathway) that are organized in homologous ways. These pathways are conserved through evolutionary time, and they undergo descent with modification. Such homologies of processes become critical to the discussion of evolution and development when we consider (1) that evolution depends on heritable changes in development, (2) that development is modular such that different modules can change without affecting other modules, (3) that modules can be co-opted into new functions, and (4) that modules depend on intercellular communication.

Animals↗

PubMed1996

Developmental genetics and traditional homology.

The concept of homology arose from classical studies of comparative morphology, and took on a new significance with the advent of evolutionary theory. It is currently undergoing another metamorphosis: many developmental geneticists now define homology as shared patterns of gene expression. However, this new usage conflates definition with criteria, and fails to recognize that meaningful assignments of homology must specify a biological level. We argue that although developmental genetic data can help identify homologous structures, they are neither necessary nor sufficient, and do not in any case justify a new definition of homology.

Anatomy, Comparative↗

PubMed1995

Model systems in developmental biology.

The practical criteria by which developmental biologists choose their model systems have evolutionary correlates. The result is a sample that is not merely small, but biased in particular ways, for example towards species with rapid, highly canalized development. These biases influence both data collection and interpretation, and our views of how development works and which aspects of it are important.

Animals↗

PubMed1993

Gastrulation and mesoderm morphogenesis in the white sturgeon.

This study presents a detailed description of gastrulation in the white sturgeon, Acipenser transmontanus, using scanning electron microscopy, histology, and time-lapse filming and video microscopy. This morphological analysis describes the similarity of gastrula structure in the sturgeon and the amphibian Xenopus laevis, and suggests that the species share many developmental mechanisms. It also identifies important differences, such as the equatorial dorsal lip in sturgeon, and provides a basis for interpreting experiments that test the effect of these differences on gastrulation. The onset of gastrulation in the sturgeon is marked by the appearance of a blastoporal equatorial pigment line that forms as the apices of bottle cells contract and concentrate surface pigment granules. Bottle cell formation at the blastopore lip and involution of surface material through the blastopore are strikingly similar to the equivalent processes in amphibian embryos. As gastrulation continues, a distinct cleft of Brachet forms between pre-involution and post-involution material. Following involution, the prospective axial mesoderm located on the dorsal surface of the late blastula (Ballard and Ginsburg: J. Exp. Zool., 213:69-103, 1980) ingresses from a central zone in the posterior archenteron roof surface in a process that is unlike any in Xenopus, but resembles events in other amphibians (Purcell, 1992; Smith: Dev. Biol., 98:250-254, 1983; King: Biol. Bull., 4:287-300, 1903). The detailed comparison of similarities and differences in gastrulation in different vertebrate lineages yields insights into the function and versatility of common developmental mechanisms.

Animals↗

PubMed1993

The mechanism of gastrulation in the white sturgeon.

Gastrulation in the white sturgeon, Acipenser transmontanus, involves many of the same processes as in the amphibian Xenopus laevis, but the timing and relative importance of these processes are altered so that they function appropriately in a different type of egg. In both species, convergence and extension result from a combination of radial and mediolateral cell intercalation. In sturgeons, where the blastopore lip forms at the equator, an early phase of thinning and extension of the animal cap moves the marginal zone below the equator during late blastula and early gastrula stages. This early extension without convergence is followed by convergent extension of the dorsal marginal zone after its displacement vegetally. When the animal cap is removed before gastrulation, precluding the initial extension that moves the marginal zone below the equator, autonomous convergence of the lower marginal zone produces an equatorially constricted embryo. Dorsal explants of sturgeon embryos undergo convergent extension similar to that documented in Xenopus (Keller and Danilchik: Development, 103:193-209, 1988), with distinct zones of extension in the involuting and non-involuting marginal zone regions. The extension of cultured explants demonstrates that this morphogenetic behavior is intrinsic to the dorsal tissue. These results show that normal gastrulation depends not only on the function of these independent morphogenetic mechanisms, but also on their mechanical context in the embryo. Experimental analyses and comparison of gastrulation in similar embryos, such as those of Xenopus and sturgeons, reveal both common developmental mechanisms, and variation in their roles.

Animals↗