Evolution of predetermined germ cells in vertebrate embryos: implications for macroevolution

The germ line is established in animal embryos with the formation of primordial germ cells (PGCs), which give rise to gametes. Therefore, the need to form PGCs can act as a developmental constraint by inhibiting the evolution of embryonic patterning mechanisms that compromise their development. Conversely, events that stabilize the PGCs may liberate these constraints. Two modes of germ cell determination exist in animal embryos: (a) either PGCs are predetermined by the inheritance of germ cell determinants (germ plasm) or (b) PGCs are formed by inducing signals secreted by embryonic tissues (i.e., regulative determination). Surprisingly, among the major extant amphibian lineages, one mechanism is found in urodeles and the other in anurans. In anuran amphibians PGCs are predetermined by germ plasm; in urodele amphibians PGCs are formed by inducing signals. To determine which mechanism is ancestral to the tetrapod lineage and to understand the pattern of inheritance in higher vertebrates, we used a phylogenetic approach to analyze basic morphological processes in both groups and correlated these with mechanisms of germ cell determination. Our results indicate that regulative germ cell determination is a property of embryos retaining ancestral embryological processes, whereas predetermined germ cells are found in embryos with derived morphological traits. These correlations suggest that regulative germ cell formation is an important developmental constraint in vertebrate embryos, acting before the highly conserved pharyngula stage. Moreover, our analysis suggests that germ plasm has evolved independently in several lineages of vertebrate embryos.     

MULTIVARIATE PHENOTYPIC DIFFERENTIATION AMONG BOTTLENECK LINES OF THE HOUSEFLY

Multivariate phenotypic differentiation in eight morphometric traits was examined in bottleneck lines of the housefly initiated with one, four, or 16 pairs of flies from a natural outbred population. Differentiation was assessed using a Mahalanobis' distance metric in units of additive genetic variance and covariance estimated from the ancestral population (i.e., generalized genetic distance). This distance metric was partitioned into contributions of size and shape to total distance. Bottleneck lines of all sizes diverged significantly from the ancestral line, but the direction of these shifts differed among the lines of different initial founding size. Those populations founded with single pairs diverged from the ancestral line mostly in shape; the 16-pair lines differentiated almost entirely in size, and the four-pair lines were intermediate in the relative contribution of shape to differentiation from the control. Bottlenecks serve to alter the genetic relationships among traits within the derived populations and in doing so could promote speciation by permitting differentiation of the populations along evolutionary trajectories less accessible to the base population.     

Inferences on the role of insertion in a mutation accumulation experiment with Drosophila melanogaster using RAPDs

The genetic variability for RAPDs band pattern was studied in a set of 157 mutation accumulation (MA) lines of Drosophila melanogaster. These MA lines were derived from the same isogenic base population and subsequently maintained by full-sib mating during 132 generations. The ancestral pattern of the original isogenic base can be unambiguously established as the consensus pattern of the MA lines and, because these lines are expected to be homozygous, dominance for band pattern is not a concern. Only repeatable changes in band pattern were considered. The number of ancestral bands detected implies that nine-nucleotide targets are enough for repeatable PCR amplification. Compared with the ancestral pattern, one MA line lost one band and two MA lines gained a new one. These results can be accounted for by the insertion of transposable elements occurring at a rate 0.07 < i < 0.21 per whole haploid genome and generation. This range is typical for Drosophila and consistent with the previously observed mobility for the roo family, supporting the generality of previous estimates of spontaneous mutation rates for morphological and fitness traits based on these MA lines. The sequence of one of the new bands suggests that the Idefix family is also active in the lines.     

Live history evolution in Serpulimorph polychaetes: a phylogenetic analysis

The widely accepted hypothesis of plesiomorphy of planktotrophic, and apomorphy of lecithotrophic, larval development in marine invertebrates has been recently challenged as a result of phylogenetic analyses of various taxa. Here the evolution of planktotrophy and lecithotrophy in Serpulimorph polychaetes (families Serpulidae and Spirorbidae) was studied using a hypothesis of phylogenetic relationships in this group. A phylogenetic (parsimony) analysis of 36 characters (34 morphological, 2 developmental) was performed for 12 selected serpulid and 6 spirorbid species with known reproductive/developmental strategies. Four species of Sabellidae were used in the outgroup. The analysis yielded 4 equally parsimonious trees of 78 steps, with a consistency index (CI) of 0.654 (CI excluding uninformative characters is 0.625). Under the assumption of unweighted parsimony analysis, planktotrophic larvae are apomorphic and non-feeding brooded embryos are plesiomorphic in serpulimorph polychaetes. The estimated polarity of life history transitions may be strengthened by further studies demonstrating an absence of a unidirectional bias in planktotrophy-lecithotrophy transition in polychaetes.     

Evolutionary implications of the mode of D quadrant specification in coelomates with spiral cleavage

In annelids, molluscs, echiurans and sipunculids the establishment of the dorsal-ventral axis of the embryo is associated with D quadrant specification during embryogenesis. This specification occurs in two ways in these phyla. One mechanism specifies the D quadrant via the shunting of a set of cytoplasmic determinants located at the vegetal pole of the egg to one blastomere of the four cell stage embryo. In this case, at the first two cleavages of embryogenesis there is an unequal distribution of cytoplasm, generating one macromere which is larger than the others at the four cell stage. The D quadrant can also be specified by a contact mediated inductive interaction between one of the macromeres at the vegetal pole with micromeres at the animal pole of the embryo. This mechanism operates at a later stage of development than the cytoplasmic localization mechanism and is associated with a pattern of cleavage in which the first two cleavages are equal. An analysis of the phylogenetic relationships within these phyla indicates that the taxa which determine the D quadrant at an early cleavage stage by cytoplasmic localization tend to be derived and lack a larval stage or have larvae with adult characters. Those taxa where the D quadrant is specified by induction include the ancestral groups although some derived groups also use this mechanism. The pulmonate mollusc Lymnaea uses an inductive mechanism for specifying the D quadrant. In these embryos each of the four vegetal macromeres has the potential of becoming the D macromere; however under normal circumstances one of the two vegetal crossfurrow macromeres almost invariably becomes the D quadrant. Experiments are described here in which the size of one of the blastomeres of the four cell stage Lymnaea embryo is increased; this macromere invariably becomes the D quadrant. These experiments suggest that developmental change in relative blastomere size during the first two cleavages in spiralian embryos that normally cleave equally may have provided a route that has led to the establishment of the cytoplasmic localization mechanism of D quadrant formation.     

The role of cell lineage in development

Studies of the role of cell lineage in development began in the latter part of the 19th century, fell into decline in the early part of the 20th, and were revived about 20 years ago. This recent revival was accompanied by the introduction of new and powerful analytical techniques. Concepts of importance for cell lineage studies include the principal division modes by which a cell may give rise to its descendant clone (proliferative, stem cell and diversifying); developmental determinacy, or indeterminacy, which refer to the degree to which the normal cleavage pattern of the early embryo and the developmental fate of its individual cells is, or is not, the same in specimen after specimen; commitment, which refers to the restriction of the developmental potential of a pluripotent embryonic cell; and equivalence group, which refers to two or more equivalently pluripotent cell clones that normally take on different fates but of which under abnormal conditions one clone can take on the fate of another. Cell lineage can be inferred to have a causative role in developmental cell fate in embryos in which induced changes in cell division patterns lead to changes in cell fate. Moreover, such a causative role of cell lineage is suggested by cases where homologous cell types characteristic of a symmetrical and longitudinally metameric body plan arise via homologous cell lineages. The developmental pathways of commitment to particular cell fates proceed according to a mixed typologic and topographic hierarchy, which appears to reflect an evolutionary compromise between maximizing the ease of ordering the spatial distribution of the determinants of commitment and minimizing the need for migration of differentially committed embryonic cells. Comparison of the developmental cell lineages in leeches and insects indicates that the early course of embryogenesis is radically different in these phyletically related taxa. This evolutionary divergence of the course of early embryogenesis appears to be attributable to an increasing prevalence of polyclonal rather than monoclonal commitment in the phylogenetic line leading from an annelid-like ancestor to insects.     

Emerging systems: between vertebrates and arthropods,the Lophotrochozoa

Novel molecular model organisms for the study of development and regeneration are emerging among the Lophotrochozoa, the third major branch of bilaterian animals. The polychaete Platynereis, the leech Helobdella, the snail Ilyanassa, and several planarians are efficiently accessed for molecular techniques including large-scale whole-mount in situ hybridization screening, RNA interference or morpholino knock-down. Joint efforts include the generation of genomic resources in the form of expressed sequence tag collections and bacterial artificial chromosome libraries. Current research focuses on early pattern formation during cleavage, the emergence and diversification of body segments, and the formation of photoreceptor cells and eyes. Several lophotrochozoan groups (in particular nereid polychaetes) exhibit modes of development, organ design, or body plans that are considered ancestral in many respects. This is also reflected in the level of genes, making these groups ideally suited for developmental comparative studies.     

Cell lineage,axis formation,and the origin of germ layers in the amphipod crustacean Orchestia cavimana

Embryos of the amphipod crustacean Orchestia cavimana are examined during cleavage, gastrulation, and segmentation by using in vivo labelling. Single blastomeres of the 8- and 16-cell stages were labelled with DiI to trace cell lineages. Early cleavage follows a distinct pattern and the a/p and d/v body axes are already determined at the 4- and 8-cell stages, respectively. In these stages, the germinal rudiment and the naupliar mesoderm can be traced back to a single blastomere each. In addition, the ectoderm and the postnaupliar mesoderm are separated into right and left components. At the16-cell stage, naupliar ectoderm is divided from the postnaupliar ectoderm, and extraembryonic lineages are separated from postnaupliar mesoderm and endoderm. From our investigation, it is evident that the cleavage pattern and cell lineage of Orchestia cavimana are not of the spiral type. Furthermore, the results of the labelling show many differences to cleavage patterns and cell lineages in other crustaceans, in particular, other Malacostraca. The cleavage and cell lineage patterns of the amphipod Orchestia are certainly derived within Malacostraca, whose ancestral cleavage mode was most likely of the superficial type. On the other hand, Orchestia exhibits a stereotyped cell division pattern during formation and differentiation of the germ band that is typical for malacostracans. Hence, a derived (apomorphic) early cleavage pattern is the ontogenetic basis for an evolutionarily older cell division pattern of advanced developmental stages. O. cavimana offers the possibility to trace the lineages and the fates of cells from early developmental stages up to the formation of segmental structures, including neurogenesis at a level of resolution that is not matched by any other arthropod system.     

Inferring the ancestral sexuality and reproductive condition in sponges (Porifera)

Considerable diversity abounds among sponges with respect to reproductive and developmental biology. Their ancestral sexual mode (gonochorism vs. hermaphroditism) and reproductive condition (oviparity vs. viviparity) however remain unclear, and these traits appear to have undergone correlated evolution in the phylum. To infer ancestral traits and investigate this putative correlation, we used DNA sequence data from two loci (18S ribosomal RNA and cytochrome c oxidase subunit I) to explore the phylogenetic relationships of 62 sponges whose reproductive traits have been previously documented. Although the inferred tree topologies, using the limited data available, favoured paraphyly of sponges, we also investigated ancestral character‐state reconstruction on a phylogeny with constrained sponge monophyly. Both parsimony‐ and likelihood‐based ancestral state reconstructions indicate that viviparity (brooding) was the likely reproductive mode of the ancestral sponge. Hermaphroditism is favoured over gonochorism as the sexual condition of the sponge ancestor under parsimony, but the reconstruction is ambiguous under likelihood, rendering the ancestry of sexuality unresolved in our study. These results are insensitive to the constraint of sponge monophyly when tracing the reproductive characters using parsimony methods. However, the maximum likelihood analysis of the monophyletic hypothetical tree rendered gonochorism as ancestral for the phylum. A test of trait correlation unambiguously favours the concerted evolution of sexuality and reproductive mode in sponges (hermaphroditism/viviparity, gonochorism/oviparity). Although testing ecological hypotheses for the pattern of sponge reproduction is beyond the scope of our analyses, we postulate that certain physiological constrains might be key causes for the correlation of reproductive characters.     

Phylogenetic analysis of developmental and postnatal mouse cell lineages

SUMMARY Fate maps depict how cells relate together through past lineage relationships, and are useful tools for studying developmental and somatic processes. However, with existing technologies, it has not been possible to generate detailed fate maps of complex organisms such as the mouse. We and others have therefore proposed a novel approach, "phylogenetic fate mapping," where patterns of somatic mutation carried by the individual cells of an animal are used to retrospectively deduce lineage relationships through phylogenetic inference. Here, we have cataloged genomic polymorphisms at 324 mutation-prone polyguanine tracts for nearly 300 cells isolated from a single mouse, and have explored the cells' lineage relationships both phylogenetically and through a network-based approach. We present a model of mouse embryogenesis, where an early period of substantial cell mixing is followed by more coherent growth of clones later. We find that cells from certain tissues have greater numbers of close relatives in other specific tissues than expected from chance, suggesting that those populations arise from a similar pool of ancestral lineages. Finally, we have investigated the dynamics of cell turnover (the frequency of cell loss and replacement) in postnatal tissues. This work offers a longitudinal study of developmental lineages, from conception to adulthood, and provides insight into basic questions of mouse embryology as well as the somatic processes that occur after birth.     

The Influence of Phylogeny on the Social Behaviour of Macaques (Primates: Cercopithecidae, genus Macaca)

The macaques are the most geographically widespread and behaviourally diverse primate genus. Many of the diversified features of social behaviour and organization in this genus have been attributed to socio-ecological differences. However, some core aspects of social behaviour and organization appear to be highly conservative, suggesting a high degree of phylogenetic inertia. A recently derived classification of macaque social organization, as well as a large range of social and physical traits, were analysed for 16 species to ascertain the role of phylogeny in explaining the distribution of these traits within the genus Macaca . These traits were mapped onto two alternative phylogenies. The pattern of social organization, based on the degree of asymmetry in social relations, exhibited a high and significant level of phylogenetic inertia, as did seven of the 22 individual traits compared. A profile constructed of the most likely ancestral state for this character matrix showed that it most closely resembles the pattern present in the Barbary macaque ( M. sylvanus ), which, based on both molecular and morphological data, most closely resembles the ancestral macaque.     

Phylogenetic fate mapping: theoretical and experimental studies applied to the development of mouse fibroblasts

Mutations are an inevitable consequence of cell division. Similarly to how DNA sequence differences allow inferring evolutionary relationships between organisms, we and others have recently demonstrated how somatic mutations may be exploited for phylogenetically reconstructing lineages of individual cells during development in multicellular organisms. However, a problem with such "phylogenetic fate maps" is that they cannot be verified experimentally; distinguishing actual lineages within clonal populations requires direct observation of cell growth, as was used to construct the fate map of Caenorhabditis elegans, but is not possible in higher organisms. Here we employ computer simulation of mitotic cell division to determine how factors such as the quantity of cells, mutation rate, and the number of examined marker sequences contribute to fidelity of phylogenetic fate maps and to explore statistical methods for assessing accuracy. To experimentally evaluate these factors, as well as for the purpose of investigating the developmental origins of connective tissue, we have produced a lineage map of fibroblasts harvested from various organs of an adult mouse. Statistical analysis demonstrates that the inferred relationships between cells in the phylogenetic fate map reflect biological information regarding the origin of fibroblasts and is suggestive of cell migration during mesenchymal development.     

The importance of colony structure versus shoot morphology for the water balance of 22 subarctic bryophyte species

Questions: What are the water economy strategies of the dominant subarctic bryophytes in terms of colony and shoot traits? Can colony water retention capacity be predicted from morphological traits of both colonies and separate shoots? Are suites of water retention traits consistently related to bryophyte habitat and phylogenetic position? Location: Abisko Research Station, North Sweden. Methods: We screened 22 abundant subarctic bryophyte species from diverse habitats for water economy traits of shoots and colonies, including desiccation rates, water content at field capacity, volume and density (mg cm^?3) of water‐saturated and oven‐dried patches, evaporation rate (g·m^?2·s^?1) and cell wall thickness. The relationships between these traits and shoot and colony desiccation rates were analysed with Spearman rank correlations. Subsequent multivariate (cluster followed by PCA) analyses were based on turf density, turf and shoot desiccation rate, cell wall thickness and amount of external and internal water. Results: Individual shoot properties, i.e. leaf cell wall properties, water retention capacity and desiccation rate, did not correspond with colony water retention capacity. Colony desiccation rate depended on density of water‐saturated colonies, and was marginally significantly negatively correlated with species individual shoot desiccation rate but not related to any other shoot or colony trait. Multivariate analyses based on traits assumed to determine colony desiccation rate revealed six distinct species groups reflecting habitat choice and phylogenetic relationships. Conclusions: General relationships between shoot and colony traits as determinants of water economy will help to predict and upscale changes in hydrological function of bryophyte‐dominated peatlands undergoing climate‐induced shifts in species abundance, and feedbacks of such species shifts on permafrost insulation and carbon sequestration functions.     

Cell clones and pattern formation: Developmental restrictions in the compound eye ofDrosophila

Summary Five regions of the compound eye have been found to be preferential boundaries for clones of labelledMinute ⁺ cells, and to act restrictively on the growth of cell clones after a given developmental stage. One of these regions is topographically related to the line of pattern inversion existing at the level of the equator. The results of experiments showing independency of origin of restriction lines and line of pattern inversion are reported.     

Primitive versus derived traits in the developmental program of the vertebrate head: views from cyclostome developmental studies

Evolution can be viewed as a series of changes in the developmental program along the phylogenetic tree. To better understand the early evolution of the vertebrate skull, we can use the embryos of the cyclostome species as models. By comparing the cyclostome developmental patterns with those of gnathostomes, it becomes possible to distinguish the primitive and derived parts of the developmental program as taxon-specific traits. These traits are often recognizable as developmental constraints that define taxa by biasing the developmental trajectories within a certain limited range, resulting in morphological homologies in adults. These developmental constraints are distributed on the phylogenetic tree like the morphological character states of adult animals and are associated with specific regions of the tree. From this perspective, we emphasize the importance of considering gene expression and embryonic anatomy as the mechanistic bases that can result in homologous or nonhomologous morphological patterns at later developmental stages. Taking the acquisition of the jaw and trabecula cranii as examples, we demonstrate that a set of embryonic features can be coupled or decoupled during evolution and development. When they are coupled, they exert an ancestral developmental constraint that results in homologous morphological patterns, and when they are decoupled, the ancestral constraints tend to be abandoned, generating a new body plan. The heterotopy behind the specification of the oral domain is an example of decoupling, based on shifted tissue interactions. We also stress the importance of "developmental burden" in determining the sequential order of changes through evolution.     

Phylogenetic positions of the aberrant branchiobdellidans and aphanoneurans within the Annelida as derived from 18S ribosomal RNA gene sequences

Different hypotheses have been proposed on the phylogenetic relationships of branchiobdellidans and aphanoneurans among the Annelida based on the anatomical and embryological characters. The 18S ribosomal RNA gene sequences have been analyzed from representatives of the three major taxa of the Annelida plus the branchiobdellidans and aphanoneurans to assess their phylogenetic relationships to each other. In this preliminary study, all of the phylogenetic analyses show the branchiobdellidans as a sister group to the leeches, rather than the oligochaetes. The position of the aphanoneurans is stable as an independent taxon that evolved after the polychaetes branched from the evolutionary stem, but before the ancestral oligochaetes emerged.     

Ancient Wings: animating the evolution of butterfly wing patterns

Character optimization methods can be used to reconstruct ancestral states at the internal nodes of phylogenetic trees. However, seldom are these ancestral states visualized collectively. Ancient Wings is a computer program that provides a novel method of visualizing the evolution of several morphological traits simultaneously. It allows users to visualize how the ventral hindwing pattern of 54 butterflies in the genus Bicyclus may have changed over time. By clicking on each of the nodes within the evolutionary tree, the user can see an animation of how wing size, eyespot size, and eyespot position relative the wing margin, have putatively evolved as a collective whole. Ancient Wings may be used as a pedagogical device as well as a research tool for hypothesis-generation in the fields of evolutionary, ecological, and developmental biology.     

Progenetic species in polychaetes (Annelida) and problems assessing their phylogenetic affiliation

Progenesis is defined as the retention of ancestral juvenilecharacters by adult stages of descendants due to an accelerationof the sexual maturation and thus is often regarded as a fastevolutionary process. Several small, meiofaunal polychaetes,such as Dinophilidae, some "Dorvilleidae" (for example, Parapodrilus),and Protodrilida, exhibit morphological simplicity in that theylack features typical of larger polychaetes, for example, parapodiaand/or head appendages. Due to the general resemblance of adultmeiofaunal polychaetes to juveniles of larger forms, progenesishas been invoked to explain evolutionary origins of many smallertaxa with increasing frequency over the past 4 decades. In thisreview, I summarize the interstitial species of polychaetesfor which progenetic origin has been suggested on the basisof morphology. However, critical examination of morphologicaldata that includes larval features reveals that autapomorphiccharacters uniting supposed progenetic taxa to specific annelidlineages are often missing. Typically larval and juvenile characters,which are argued to support hypotheses of progenetic origin,are often widely dispersed, homoplastic features. Because ofthis situation, molecular data seem to be the most reliablesource for phylogenetic inference. However, other biologicaldata, for example, from life history and morphology, are necessitiesto substantiate the progenetic evolution of these species.