Valerie B. Morris

Regulatory punctuated equilibrium and convergence in the evolution of developmental pathways in direct-developing sea urchins

Summary We made hybrid crosses between closely and distantly related sea urchin species to test two hypotheses about the evolution of gene regulatory systems in the evolution of ontogenetic pathways and larval form. The first hypothesis is that gene regulatory systems governing development evolve in a punctuational manner during periods of rapid morphological evolution but are relatively stable over long periods of slow morphological evolution. We compared hybrids between direct and indirect developers from closely and distantly related families. Hybrids between eggs of the direct developer Heliocidaris erythrogramma and sperm of the 4‐million year distant species H. tuberculata, an indirect developer, restored feeding larval structures and paternal gene expression that were lost in the evolution of the direct‐developing maternal parent. Hybrids resulting from the cross between eggs of H. erythrogramma and sperm of the 40‐million year distant indirect‐developer Pseudoboletia maculata are strikingly similar to hybrids between the congeneric hybrids. The marked similarities in ontogenetic trajectory and morphological outcome in crosses of involving either closely or distantly related indirect developing species indicates that their regulatory mechanisms interact with those of H. erythrogramma in the same way, supporting remarkable conservation of molecular control pathways among indirect developers. Second, we tested the hypothesis that convergent developmental pathways in independently evolved direct developers reflect convergence of the underlying regulatory systems. Crosses between two independently evolved direct‐developing species from two 70‐million year distant families, H. erythrogramma and Holopneustes purpurescens, produced harmoniously developing hybrid larvae that maintained the direct mode of development and did not exhibit any obvious restoration of indirect‐developing features. These results are consistent with parallel evolution of direct‐developing features in these two lineages.

Evolutionary Modification of Echinoid Sperm Correlates with Developmental Mode

A significant fraction of living sea urchin species have completely or partially eliminated the pluteus larval stage and instead develop directly from embryo to adult. Direct developing sea urchins develop from large buoyant eggs. We present data to show that evolution of these large eggs is accompanied by the evolution of spermatozoa with elogate heads, in contrast with the conical sperm heads typical of most echinoids. Two congeneric Australian species, Heliocidaris tuberculata, which develops via a pluteus, and H. erythogramma, a direct developer, were investigated in detail. The sperm of H. erythrogramma have an elongate head (11 μm in length) as compared to the conical sperm head (5.6 μm) of H. tuberculata. Electrophoretic analysis of the sperm histones indicates that no unusual histones or protamines are associated with modified head morphology. Genome sizes were determined by flow cytometry. H. erythrogramma has a haploid genome size of 1.3 pg as compared to a haploid genome size of 0.95 pg for H. tuberculata. Other direct developing echinoids have elongate sperm heads, and co‐evolution of gametes is indicated as a common feature of evolution of direct development in echinoids. The most extreme case, the direct developing cidaroid sea urchin, Phyllacanthus parvispinus, possesses the longest and narrowest sperm head (20 μm × 1 μm) ever observed in an echinoid.

Division rules for polygonal cells

A number of fascinating mathematical problems concerning the division of two-dimensional space are formulated from questions about the planes of cell division in embryonic epithelia. Their solution aids in the quantitative description of cellular arrangement in epithelia. Cells, considered as polygons, site their division line according to stochastic rules, eventually forming a tessellation of the plane. The equilibrium distributions for the resulting mix of polygonal types are explored for a range of stochastic rules. We find surprising links with some classical distributions from the theory of probability.

Random segregation of sister chromatids in developing chick retinal cells demonstrated in vivo using the fluorescence plus Giemsa technique.

Experiments were designed to test whether nonrandom segregation of sister chromatids at mitosis has a role in the production of cell diversity during embryogenesis. Segregation was examined in vivo in retinal cells from embryonic chicks. Chromatids were labelled with bromouracil and stained by the fluorescence plus Giemsa technique. No evidence of nonrandom segregation was observed in a frequency distribution of pairs of bifilarly labelled sister chromatids at the third metaphase after the start of labelling. Nor was there evidence that chromatids from homologous chromosomes segregated nonrandomly. Nonrandom segregation is probably not a mechanism for cell diversification.

A Growth Curve of Cell Numbers In the Neural Retina of Embryonic Chicks

ABSTRACT The growth of cell numbers in a normal embryonic population of ncural retinal cells is described. the numbers were estimated from a time shortly after the neural retina first becomes recognizable to a time when numbers of retinal cells have become steady. Cell numbers were estimated in preparations of an entire neural retina dispersed into a suspension of single nuclei which were then counted in a Coulter counter. the growth curve of the In numbers of cells has three phases of growth: an exponential phase during which there is steady‐state exponential growth, a differentiative phase during which cell proliferation ceases and an end phase when no further change in cell numbers can be detected. the variances of the In numbers of cells were highest during the exponential phase. the variances decreased during the differentiative phase and were at their lowest during the end phase. For variances to decrease requires mechanisms which control the final numbers of cells in the neural retina very precisely. the implications of mechanisms which operate by controlling cell lineages are explored.

A method for the measurement of variability in cell lifetimes

Abstract The probability distributions of cell lifetime (cycle time) and phase durations are of considerable interest. Various experimental methods have been devised to collect data concerning these distributions. In this paper we present a mathematical analysis for such an experiment where cells are labeled with 5-bromodeoxyuridine during DNA synthesis (S phase) and observed at a later time during mitosis. The labeling technique gives information about the number of S phases encountered by the cell in the intervening period. The technique is a considerable refinement on the so-called fraction-labeled mitosis experiments, since in those experiments the labeling gives information only about whether or not an S phase is encountered.

HOX-TYPE AND NON-HOX HOMEOBOX GENE SEQUENCES IN GENOMIC DNA OF THE SEA URCHIN HOLOPNEUSTES PURPURESCENS

As a preliminary step in an analysis of Hox gene expression and radial body plan specification in sea urchin development, we amplified partial homeobox sequences in H. purpurescens by PCR using degenerate primers. The primers, HoxE and HoxF (Pendleton et al., 1993), spanned a highly conserved region of 82 nucleotides encompassing amino acids 21-47 of the homeodomain. Seven Hox-type homeobox sequences and two non-Hox homeobox sequences were identified. The seven Hox-type sequences were placed provisionally in Hox paralogous groups, one in paralogous group 3, three in paralogous groups 6-8 and three in paralogous groups 9 13. The non-Hox sequences had similarities with Xlox and Gbx homeobox genes.

An analysis of the growth of the retinal cell population in embryonic chicks yielding proliferative ratios, numbers of proliferative and non‐proliferative cells and cell‐cycle times for successive generations of cell cycles

Growth curves of the retinal cell population of embryonic chicks were fitted by a branching‐process model of cell population growth, thereby estimating the proliferative ratios and mean cell‐cycle times of the generations of cell cycles that underlie retinal growth. The proliferative ratio determines the proportion of cells that divides in the next generation, so the numbers of proliferative and non‐proliferative cells in each generation of cell cycles were obtained. The mean cell‐cycle times determine the times over which the generations are extant. Assuming growth starts from one cell in generation 0, the proliferative cells reach 3.6 × 106 and the non‐proliferative cells reach 1.1 × 106 by generation 23. The next four generations increase the proliferative cell numbers to 13.9 × 106 and produce 20.1 × 106 non‐proliferative cells. In the next five generations in the end phase of growth, non‐proliferative cells are produced in large numbers at an average of 13.9 × 106 cells per generation as the retinal lineages are completed. The retinal cell population reaches a maximum estimated here at 98.2 × 106 cells. The mean cell‐cycle time estimates range between 6.8 and 10.1 h in generations before the end phase of growth and between 10.6 and 17.2 h in generations in the end phase. The retinal cell population growth is limited by the depletion of the proliferative cell population that the production of non‐proliferative cells entails. The proliferative ratios and the cell‐cycle‐time distribution parameters are the likely determinants of retinal growth rates. The results are discussed in relation to other results of spatial and temporal patterns of the cessation of cell cycling in the embryonic chick retina.

Development of the five primary podia from the coeloms of a sea star larva: homology with the echinoid echinoderms and other deuterostomes

Confocal laser scanning microscopy of larvae of the asteroid Parvulastra exigua was used to investigate the development of the five primary podia from the coeloms in the echinoderm phylum in an approach to the problem of morphological homology in the deuterostome phyla. The development is shown from an early brachiolaria larval stage to a pre-settlement late brachiolaria larval stage. In the early brachiolaria larva, a single enterocoele connected to the archenteron has formed into two lateral coeloms and an anterior coelom. The primary podia form from the coelomic regions on the left side of the brachiolaria larva, while on the right the coelomic regions connect with the exterior through the pore canal and hydropore. The anterior coelom forms the coelom of the brachia. Homology between the primary podia of the asteroid and the echinoid classes of echinoderms is described and extended to coeloms of other deuterostome phyla.

Cell population dynamics during the differentiative phase of tissue development

The dynamics of a cell population whose numbers are growing exponentially have been described well by a mathematical model based on the theory of age-dependent branching processes. Such a model, however, does not cover the period following exponential growth when cell differentiation curtails population size. This paper offers an extension to the branching process model to remedy this deficiency. The extended model is ideal for describing embryonic growth; its use is illustrated with data from embryonic retina. The model offers a better computational framework for the interpretation of a variety of data (growth curves of cell numbers, DNA histograms, thymidine labelling indices, FLM curves, BUdR-labelled mitoses curves) because age-distributions can be calculated at any stage of development, not just during exponential growth. Proportions of cells in the various phases of the cell cycle can be computed as growth slows. Such calculations show the gradual transition from a population dominated by cells which are young with respect to cell cycle age to one dominated by those which are old, and the effects such biases have on the proportions of cells in each phase.