Adaptive evolution of larvae and life cycles

The larval patterns of marine invertebrates pose intriguing questions for both evolutionary and developmental biologists. However, combined investigations have been rare. Quantitative models analyze the selective factors that drive evolutionary change in larval nutrition and timing of metamorphosis. Developmental studies describe the morphogenesis characterizing ancestral and derived larval patterns. Rigorous evolutionary analysis of the transition to derived modes of development is lacking and detailed developmental and ecological data are needed to test and refine theoretical models. A major challenge facing studies of life cycle evolution is the elucidation of the genetic structure and covariance of important developmental and larval traits.     

Assembling the lophotrochozoan (=spiralian) tree of life

The advent of numerical methods for analysing phylogenetic relationships, along with the study of morphology and molecular data, has driven our understanding of animal relationships for the past three decades. Within the protostome branch of the animal tree of life, these data have sufficed to establish its two main side branches, the moulting Ecdysozoa and the non-moulting Lophotrochozoa. In this review, I explore our current knowledge of protostome relationships and discuss progress and future perspectives and strategies to increase resolution within the main lophotrochozoan clades. Novel approaches to coding morphological characters are needed by scoring real observations on species selected as terminals. Still, methodological issues, for example, how to deal with inapplicable characters or the coding of absences, may require novel algorithmic developments. Taxon sampling is another key issue, as phyla should include enough species so as to represent their span of anatomical disparity. On the molecular side, phylogenomics is playing an increasingly important role in elucidating animal relationships, but genomic sampling is still fairly limited within the lophotrochozoan protostomes, for which only three phyla are represented in currently available phylogenies. Future work should therefore concentrate on generating novel morphological observations and on producing genomic data for the lophotrochozoan side of the animal tree of life.     

The animal in the genome: comparative genomics and evolution

Comparisons between completely sequenced metazoan genomes have generally emphasized how similar their encoded protein content is, even when the comparison is between phyla. Given the manifest differences between phyla and, in particular, intuitive notions that some animals are more complex than others, this creates something of a paradox. Simplistic explanations have included arguments such as increased numbers of genes; greater numbers of protein products produced through alternative splicing; increased numbers of regulatory non-coding RNAs and increased complexity of the cis-regulatory code. An obvious value of complete genome sequences lies in their ability to provide us with inventories of such components. I examine progress being made in linking genome content to the pattern of animal evolution, and argue that the gap between genomic and phenotypic complexity can only be understood through the totality of interacting components.     

The evolution of complex life cycles when parasite mortality is size- or time-dependent

In complex cycles, helminth larvae in their intermediate hosts typically grow to a fixed size. We define this cessation of growth before transmission to the next host as growth arrest at larval maturity (GALM). Where the larval parasite controls its own growth in the intermediate host, in order that growth eventually arrests, some form of size- or time-dependent increase in its death rate must apply. In contrast, the switch from growth to sexual reproduction in the definitive host can be regulated by constant (time-independent) mortality as in standard life history theory. We here develop a step-wise model for the evolution of complex helminth life cycles through trophic transmission, based on the approach of Parker et al. [2003a. Evolution of complex life cycles in helminth parasites. Nature London 425, 480-484], but which includes size- or time-dependent increase in mortality rate. We assume that the growing larval parasite has two components to its death rate: (i) a constant, size- or time-independent component, and (ii) a component that increases with size or time in the intermediate host. When growth stops at larval maturity, there is a discontinuous change in mortality to a constant (time-independent) rate. This model generates the same optimal size for the parasite larva at GALM in the intermediate host whether the evolutionary approach to the complex life cycle is by adding a new host above the original definitive host (upward incorporation), or below the original definitive host (downward incorporation). We discuss some unexplored problems for cases where complex life cycles evolve through trophic transmission.     

Origin and evolution of the mitochondrial aminoacyl-tRNA synthetases

Many theories favor a fusion of 2 prokaryotic genomes for the origin of the Eukaryotes, but there are disagreements on the origin, timing, and cellular structures of the cells involved. Equally controversial is the source of the nuclear genes for mitochondrial proteins, although the alpha-proteobacterial contribution to the mitochondrial genome is well established. Phylogenetic inferences show that the nuclearly encoded mitochondrial aminoacyl-tRNA synthetases (aaRSs) occupy a position in the tree that is not close to any of the currently sequenced alpha-proteobacterial genomes, despite cohesive and remarkably well-resolved alpha-proteobacterial clades in 12 of the 20 trees. Two or more alpha-proteobacterial clusters were observed in 8 cases, indicative of differential loss of paralogous genes or horizontal gene transfer. Replacement and retargeting events within the nuclear genomes of the Eukaryotes was indicated in 10 trees, 4 of which also show split alpha-proteobacterial groups. A majority of the mitochondrial aaRSs originate from within the bacterial domain, but none specifically from the alpha-Proteobacteria. For some aaRS, the endosymbiotic origin may have been erased by ongoing gene replacements on the bacterial as well as the eukaryotic side. For others that accurately resolve the alpha-proteobacterial divergence patterns, the lack of affiliation with mitochondria is more surprising. We hypothesize that the ancestral eukaryotic gene pool hosted primordial "bacterial-like" genes, to which a limited set of alpha-proteobacterial genes, mostly coding for components of the respiratory chain complexes, were added and selectively maintained.     

Reproduction in the cold: Thorson revisited

Summary

Although a tendency for high latitude marine invertebrates to avoid pelagic larval stages was first described in the 19th century, the most detailed early study was that of Thorson in Greenland. This work also established other features of the reproduction of polar marine invertebrates that have become regarded as almost axiomatic (e.g., the release of larvae to coincide with the summer bloom) or largely ignored (a latitudinal cline in egg size within species). This short and selective review examines Thorson's conclusions in the light of recent work. It is now clear that although polar prosobranch gastropods reproduce almost entirely by direct development, for many taxa the real distinction between polar and non-polar species is in the proportion of feeding to non-feeding larvae. Some species release feeding larvae in winter and the energy source for these larvae is obscure. Growth is slow and there is little or no evidence for temperature compensation. Many crustacean species have larger eggs at higher latitudes. Egg size varies significantly within species, with larger eggs being associated with larger females and often reduced fecundity. The reasons for these within-species patterns are currently unresolved.     

Origin and evolution of animal hybrid species

The increasing number of hybrid species, discovered in both vertebrates and invertebrates by the combined use of chromosome, allozyme and molecular markers, calls for a reevaluation of hybrid speciation and reticulate evolution In animals. The array of reproductive strategies recently detected In phylogenetically related stick Insects allows us to Investigate, using a comparative approach, questions such as the relationship between hybridization and unisexuality, and short- versus long-term evolutionary success of hybrid species. Unexpected similarities are now apparent in hybrid evolution of animals as varied as insects, snails, fish, frogs and lizards. Hybrid species may combine, to some extent, the main advantage of sex (genetic diversity) with those of clonal reproduction. This explains why these species are often so successful, and indicates a potential use of some hybrid species in experimental biology and resource management (e.g. mass production of animal proteins).     

Origin and evolution of Schistosoma japonicum

In his hypothesis on the coevolution of Asian schistosomes and snails, Davis implies that the ancestors of the Schistosoma japonicum and S. indicum species group were African and arrived in Asia via the Indian plate. This paper briefly reviews molecular phylogenetic relationships among species of the genus Schistosoma to test Davis’ theory about the origin and evolution of S. japonicum. All analyses using DNA base sequences, mitochondrial genome gene order and C-banding patterns suggest that Schistosoma originated in Asia and not Africa.     

Mitochondrial DNA phylogeny of North American field crickets: perspectives on the evolution of life cycles,songs, and habitat associations

North American field crickets (genus Gryllus) exhibit a diversity of life cycles, habitat associations, and calling songs. However, patterns of evolution for these ecological and behavioral traits remain uncertain in the absence of a robust phylogenetic framework. Analyses of morphological variation have provided few clues about species relationships in the genus Gryllus. Here we use comparisons of mitochondrial DNA restriction site maps for 29 individuals representing 11 species (including potential outgroups) to examine relationships among eastern North American field crickets. Initially chosen as likely outgroup taxa, the two European species of Gryllus do not obviously fall outside of an exclusively North American clade and (based on amount of sequence divergence) appear to have diverged from North American lineages at about the same time that major North American lineages diverged from each other. The egg-overwintering crickets comprise a strongly supported monophyletic group, but relationships among these three closely related species cannot be resolved. The mtDNA data are consistent with a single origin of egg diapause and do not support a model of recent life cycle divergence and allochronic speciation for Gryllus pennsylvanicus and G. veletis. The two crickets are not sister species, despite remarkable similarity in morphology, habitat, and calling song. This conclusion is consistent with published data on allozyme variation in North American field crickets. The habitat associations of eastern North American field crickets have been labile, but calling songs sometimes have remained virtually unchanged across multiple speciation events.     

蕨类植物的起源演化:对"古老"类群的重新审视

一般认为,蕨类植物是一群非常古老的植物,其演化历史可以追溯到4亿多年前。近十几年来,由于多种研究手段的应用,特别是分子系统学的异军突起,在蕨类植物起源演化方面取得了重要进展,使我们可以重新审视这一古老植物类群的演化历史。本文与传统观点相对比,简述了蕨类植物在分类范畴、起源演化及其重要类群间的系统演化关系方面的最新进展。     

Ubiquitins (polyubiquitin and ubiquitin extension protein) in marine sponges: cDNA sequence and phylogenetic analysis

The complete nucleotide sequences of two Suberites domuncula cDNAs and one Sycon raphanus cDNA, all encoding ubiquitin, have been determined. One cDNA from S. domuncula codes for polyubiquitin with four tandemly repeated monomeric units and the second cDNA encodes ubiquitin fused to a ribosomal protein of 78 amino acids (aa). S. domuncula possesses at least one additional polyubiquitin gene, from which the last two monomers were also sequenced. All analysed genes from S. domuncula encode identical ubiquitin proteins, with only one aa difference (Ala 19) to the human/higher animals ubiquitin (Pro 19). Ubiquitin in S. domuncula is identical with the ubiquitin found in another Demospongia, Geodia cydonium. The cDNA from S. raphanus encodes polyubiquitin with seven tandemly repeated units. All these gene monomers code for the same ubiquitin, which differs from the human/higher animals ubiquitin only at position 24 (Asp in Sycon, Glu in others). However, ubiquitin from S. raphanus (Calcarea) shows two aa differences (positions 19 and 24), when compared with the ubiquitin sequences from the two Demospongiae. In a phylogenetic tree constructed by multiple sequence alignment of all sponge ubiquitin gene monomers so far identified, all monomers from the same species cluster together, with the clear exception of the monomer from S. domuncula ribosomal protein fusion gene. This monomer branches off first from the tree and forms a separate line; this gives evidence for a very ancient split of ubiquitin-ribosomal-protein fusion genes from polyubiquitin encoding genes and their long separate coexistence in eukaryotes. The ubiquitin extension protein from S. domuncula is 78 aa long, displays all characteristics of 76–81 aa long ribosomal fusion proteins and shows 78% identity in the first 73 aa with the human S27a protein. However, its C-terminal sequence: 69-GLTYVYKKSD-78 is more similar to the plant consensus (69-GLTYVYQ/NK-76), than to the higher animal consensus (69-CLTYCFNK-76). This protein isolated from a sponge, belonging to the phylogenetically oldest multicellular animals, the Porifera, branches off first from the phylogenetic tree of metazoan ubiquitin extension proteins of the small ribosomal subunits.     

Phylogeny of Medusozoa and the evolution of cnidarian life cycles

To investigate the evolution of cnidarian life cycles, data from the small subunit of the ribosome are used to derive a phylogenetic hypothesis for Medusozoa. These data indicate that Cnidaria is monophyletic and composed of Anthozoa and Medusozoa. While Cubozoa and Hydrozoa are well supported clades, Scyphozoa appears to be paraphyletic. Stauromedusae is possibly the sister group of either Cubozoa or all other medusozoans. The phylogenetic results suggest that: the polyp probably preceded the medusa in the evolution of Cnidaria; within Hydrozoa, medusa development involving the entocodon is ancestral; within Trachylina, the polyp was lost and subsequently regained in the parasitic narcomedusans; within Siphonophorae, the float originated prior to swimming bells; stauromedusans are not likely to be descended from ancestors that produced medusae by strobilation; and cubozoan polyps are simplified from those of their ancestors, which possessed polyps with gastric septa and four mesogleal muscle bands and peristomial pits.     

The evolution of life cycles with haploid and diploid phases

Sexual eukaryotic organisms are characterized by an alternation between haploid and diploid phases. In vascular plants and animals, somatic growth and development occur primarily in the diploid phase, with the haploid phase reduced to the gametic cells. In many other eukaryotes, however, growth and development occur in both phases, with substantial variability among organisms in the length of each phase of the life cycle. A number of theoretical models and experimental studies have shed light on factors that may influence life cycle evolution, yet we remain far from a complete understanding of the diversity of life cycles observed in nature. In this paper we review the current state of knowledge in this field, and touch upon the many questions that remain unanswered. BioEssays 20 :453–462, 1998. © 1998 John Wiley & Sons, Inc.     

Origin and early evolution of Ceropegieae (Apocynaceae-Asclepiadoideae)

The first-branching lineages of the tribe Ceropegieae, Anisotominae, Leptadeniinae, and Heterostemminae, are the focus of the present study. In the ingroup we molecularly analysed 34 samples from southern and (north)eastern Africa and Asia for the nuclear internal transcribed spacer (ITS) region, and five plastid markers (trnT-L, trnL-F and trnH-psbA intergenic spacers, trnL, and rps16 introns). Maximum Parsimony and Bayesian analyses were conducted for phylogenetic reconstruction and Bayesian binary MCMC (BBM) analysis as implemented in RASP for testing biogeographic inference. Sister to the well-known hyperdiverse subtribe Stapeliinae (around 700 species) are the Anisotominae, an African subtribe of c. 30 species, with the tropical Neoschumannia in sister-group position to the remaining genera, Anisotoma, Emplectanthus, Riocreuxia, and Sisyranthus. Emplectanthus is sequenced in full for the first time, and its monophyly as sister to Anisotoma is documented. As in Sisyranthus, new sequences in Riocreuxia, especially in Riocreuxia torulosa s.l., reveal very low genetic variation pointing to an actively radiating and speciating group, possibly in adaptation to pollinator pressure. Molecular and morphological data necessitate the transfer of Brachystelma natalense to Anisotoma, and the new combination Anisotoma natalensis is proposed. Leptadeniinae are sister to the Stapeliinae–Anisotominae clade. The position of the strictly Asian, rheophytic Pentasachme could be resolved as sister to the remaining genera of Leptadeniinae, Conomitra, Leptadenia, and Orthanthera. The position of the S Asian/Australasian wet forest lianas of the genus Heterostemma as sister to the remaining Ceropegieae is confirmed, indicating a humid Asian origin for this tribe known for its arid-adapted African succulents.     

Origin and evolution of the parasitic cyclopoid copepods

Six of the 10 recognised families of the order Cyclopoida are parasitic, with 4 of them occurring on marine invertebrates and the remaining 2 on freshwater gastropods and fishes, respectively. A cladistic analysis of the 10 families indicates that evolution of parasitism occurred twice in the history of the cyclopoids. The first attempt was made by the marine epibenthic ancestors seeking food and shelter in sessile tunicates — the ascidians. This event led to the evolution of 2 ascidicolous families: Archinotodelphyidae and Notodelphyidae. The descendant of this lineage had also invaded the mantle cavity of marine bivalve molluscs, eventually leading to the evolution of the Mantridae. The second attempt for the parasitic mode of life was launched by the ancestor which was the sister group of the ancestral cyclopoids — the most successful family of freshwater copepods. This ancestral stock, while living in the coastal zone, split into 2 groups: one group stayed behind in the ocean and colonised again the ascidians; the other groups invaded freshwater and evolved into the fish-parasitising Lernaeidae and the gastropod-parasitising Ozmanidae.     

The life cycles and growth of Carabus glabratus and C. violaceus in Budalen, central Norway

Abstract. 1. The seasonal activity patterns of adults and larvae, and the seasonal development of female gonads showed that Curubus glabrafus Payk. and Curubus violaceus L. living in a subalpine habitat are spring (summer) breeders with biennial life cycles.
2. As a rule, the adults and third stage larvae hibernate, but C.glubrutus at least can hibernate also at the second larval stage, particularly in years in which unfavourable climatic conditions in the spring delay breeding.
3. The adaptive strategies underlying these life history patterns and their possible significance for species diversification and coexistence are discussed.