Evolutionary reorganizations of ontogenesis in related species of coenobial volvocine algae

The evolutionary aspects of ontogenesis in green volvocine algae have been considered on the basis of the author’s and published data, as well as the information on taxonomy, phylogeny, and ecology of this group. Analysis of the rate, diurnal rhythm, and light/dark control of cell divisions in various species, as well as experiments with the nucleic acid and protein synthesis inhibitors made it possible to elucidate cellular mechanisms underlying evolutionary rearrangements of asexual development in the genus Volvox.     

The evolution of developmental gene networks: lessons from comparative studies on holometabolous insects

Recent comparative studies have revealed significant differences in the developmental gene networks operating in three holometabolous insects: the beetle Tribolium castaneum, the parasitic wasp Nasonia vitripennis and the fruitfly Drosophila melanogaster. I discuss these differences in relation to divergent and convergent changes in cellular embryology. I speculate on how segmentation gene networks have evolved to operate in divergent embryological contexts, and highlight the role that co-option might have played in this process. I argue that insects represent an important example of how diversification in life-history strategies between lineages can lead to divergence in the genetic and cellular mechanisms controlling the development of homologous adult structures.     

Do Protoplasmic Connections Function in the Phototactic Coordination of the Volvox Colony During Light Stimulation?*

SYNOPSIS. The flagellar behavior of the colonial flagellates Volvox carteri Stein and Volvox perglobator Powers was examined by placing 1.01 μm polystyrene particles in solution with swimming colonies, and photographing these particle movements. When directional light stimulation was administered to individual colonies, a cessation of flagellar activity occurred in the anterior cells of the stimulated side in both species. Since Volvox perglobator possesses prominent intercellular connections and Volvox carteri does not, the results of these experiments suggest that the connections linking colony members in some species do not function in the coordination of flagellar activity associated with light orientation behavior.     

Morphogenesis in the family Volvocaceae: different tactics for turning an embryo right-side out

Green algae of the family Volvocaceae provide an unrivalled opportunity to analyze an evolutionary pathway leading from unicellularity to multicellularity with division of labor. One key step required for achieving multicellularity in this group was the development of a process for turning an embryo inside out: a morphogenetic process that is now known as "inversion," and that is a diagnostic feature of the group. Inversion is essential because at the end of its embryonic cleavage divisions, each volvocacean embryo contains all of the cells that will be present in an adult, but the flagellar ends of all cells are pointed toward the interior, rather than toward the exterior where they will need to be to function in locomotion. Inversion has been studied in greatest detail in Volvox carteri, but although all other volvocacean species have to struggle with the same awkward situation of being wrong-side out at the end of cleavage, they do it in rather different ways. Here, the inversion processes of six different volvocacean species (Gonium pectorale, Pandorina morum, Eudorina unicocca, Volvox carteri, Volvox tertius, and Volvox globator) are compared, in order to illustrate the variation in inversion patterns that exists within this family. The simplest inversion process occurs in the plate-shaped alga Gonium pectorale and the most complicated in the spherical alga Volvox globator. Gonium pectorale goes only from a concave-bowl shape to a slightly convex plate. In Volvox globator, the posterior hemisphere inverts completely before the anterior pole opens and the anterior hemisphere slides over the already-inverted posterior hemisphere; during both halves of this inversion process, the regions of maximum cell-sheet curvature move progressively, as radially symmetrical waves, along the posterior-anterior axis.     

论水螨腺毛形态学与螨类体躯进化(蜱螨亚纲：水螨群)

基于对中国水螨区系120余种的研究和有关文献,本文提出了水螨腺毛分布体位的原始模式;据腺毛腺体及围腺片等结构特征,将水螨腺毛分为四个结构类型：皱喙螨型、盾水螨型、水螨型和腺水螨型;腺毛结构特征表明,其发生与起源和体躯形态进化相关;全面评述了节肢动物和螨类体躯及体节进化的现有理论、分析讨论了关于螨类祖先体躯模式的Grandiean-Coineau氏学说(十六节说)和Bader氏学说(十二节说),由此并结合作者研究提出了螨类(水螨),原始祖先体躯模式的新假说(十八节说)。     

Cladodes,leaf-like organs in Asparagus,show the significance of co-option of pre-existing genetic regulatory circuit for morphological diversity of plants

Plants in the genus Asparagus have determinate leaf-like organs called cladodes in the position of leaf axils. Because of their leaf-like morphology, axillary position, and morphological variation, it has been unclear how this unusual organ has evolved and diversified. In the previous study, we have shown that cladodes in the genus Asparagus are modified axillary shoots and proposed a model that cladodes have arisen by co-option and deployment of genetic regulatory circuit (GRC) involved in leaf development. Moreover, we proposed that the alteration of the expression pattern of genes involved in establishment of adaxial/abaxial polarity has led to the morphological diversification from leaf-like to rod-like form of cladodes in the genus. Thus, these results indicated that the co-option and alteration of pre-existing GRC play an important role in acquisition and subsequent morphological diversification.Here, we present data of further expression analysis of A. asparagoides. The results suggested that only a part of the GRC involved in leaf development appears to have been co-opted into cladode development. Based on our study and several examples of the morphological diversification, we briefly discuss the importance of co-option of pre-existing GRC and its genetic modularity in the morphological diversity of plants during evolution.     

QUANTITATIVE PCR DATA FALSIFY THE CHROMOSOMAL ENDOREDUPLICATION HYPOTHESIS FOR VOLVOX CARTERI (VOLVOCALES, CHLOROPHYTA)

Two conflicting hypotheses for chromosome replication in the Volvocaceae, one postulating multiple rounds of replication prior to cell division (endoreduplication) and the other claiming a canonical sequence of one round of nuclear DNA replication preceding each cell division, have been tested experimentally. Competitive PCR of the single-copy actin gene (target) of Volvox carteri f. nagariensis Iyengar and a shortened gene version (competitor) containing the same primer binding sites were used to assess the genome equivalents present in a given number of cells. Determining the molar ratio of the PCR products generated from target DNA (extracted from a known number of cells) and defined numbers of competitor molecules revealed that Volvox embryos between the one- and 16-cell stages possess an average of between one and two—but never more than two—copies of the actin gene. This led us to conclude that the number of genome equivalents per nucleus in dividing Volvox embryos varies only between one and two and that, unlike the case predicted by endoreduplication, the nuclear genome undergoes only one round of replication prior to each cell division.     

Life-history evolution and the origin of multicellularity

The fitness of an evolutionary individual can be understood in terms of its two basic components: survival and reproduction. As embodied in current theory, trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. Here, we argue that the evolution of germ-soma specialization and the emergence of individuality at a new higher level during the transition from unicellular to multicellular organisms are also consequences of trade-offs between the two components of fitness-survival and reproduction. The models presented here explore fitness trade-offs at both the cell and group levels during the unicellular-multicellular transition. When the two components of fitness negatively covary at the lower level there is an enhanced fitness at the group level equal to the covariance of components at the lower level. We show that the group fitness trade-offs are initially determined by the cell level trade-offs. However, as the transition proceeds to multicellularity, the group level trade-offs depart from the cell level ones, because certain fitness advantages of cell specialization may be realized only by the group. The curvature of the trade-off between fitness components is a basic issue in life-history theory and we predict that this curvature is concave in single-celled organisms but becomes increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the initial cost of reproduction to survival which increases as group size increases. To illustrate the principles and conclusions of the model, we consider aspects of the biology of the volvocine green algae, which contain both unicellular and multicellular members.     

<Emphasis Type="Italic">Volvox</Emphasis> (Chlorophyta,Volvocales) as a model organism in developmental biology

Model systems based on two or more related species with different types of development are finding increasing use in current comparative embryology. Green algae of the genus Volvox offer an interesting opportunity to study sex pheromones, morphogenesis as well as the formation of a somatic cell line undergoing terminal differentiation, senescence, and death as well as a line of reproductive cells, which at first grow and then undergo a series of consecutive divisions that give rise to new organisms. However, almost all studies of the recent years were conducted on a single species, Volvox carteri f. nagariensis. The goal of this publication was to advertise the cosmopolitan alga V. aureus as a model species in developmental biology. Published data on V. aureus are briefly reviewed in comparison with the development of V. carteri and outlooks of further studies are specified. In particular, the expediency of collecting new V. aureus strains from nature to study their development in clonal culture is outlined.