Cladistic Approaches in the Study of Soil and Plant Parasitic Nematodes

Cladistic analysis of free-living soil nematodes of the Leptonchoidea(Nematoda: Dorylaimida) resulted in groupings different fromthose obtained by traditional methods. We can interpret distributionsof species groups obtained by phyletic analysis in relationto plate tectonic events. Similar techniques are applicableto plant parasitic nematodes. Grouping on the basis of synapomorphiesproduced a cladogram of genera of the family Heteroderidae (Nematoda:Tylenchida) in which Meloidodera and Cryphodera appear to bethe most ancestral genera and the cyst forming genera the mostderived. A cladogram of groups of species in Heterodera sensulato showed a major division, with the round cyst nematodesand the Cacli group in one grouping and the rest of the Heteroderaspecies in the second. I interpret present-day distributionsby a strict vicariance view and suggest potential falsifiers;and also discuss ancient dispersal routes as alternative waysof thinking about nematode distribution.     

Fossil primates and the evolution of some primate locomotor systems

Of Paleocene primates only Plesiadapis is complete enough to reconstruct locomotor patterns; it was an arboreal scrambler, perhaps functioning like a large squirrel. Eocene lemurs (adapids) show an array of locomotor types much like certain modern Malagasy lemurs. The European Eocene tarsiid Necrolemur and the American Hemiacodon show the beginning of saltatory specializations in possession of elongated calcaneum and astragalus. Although not a direct anthropoid ancestor Necrolemur seems one of the best models for representing the early locomotor type from which higher primates arose. The Oligocene primates of Egypt (among which are the earliest undoubted pongids) are preserved with a forest fauna. Structures of long bones suggest they were arboreal. A considerable number of Miocene ape bones are known and those of Pliopithecus and Dryopithecus indicate similar adaptations. Of African Miocene forms, Dryopithecus major was a large, gorilla-sized animal, and hence perhaps primarily terrestrial. D. africanus was somewhat more arboreally adapted and a partial brachiator. The Italian fossil Oreopithecus, a coal-swamp dweller, shows indications of bipedality in pelvic structure. Ramapithecus, which is presumably ancestral to Australopithecus, shows palatal and facial patterns much like these later hominids, and probably hence had locomotor patterns more like men than like living apes; its lack of the dental specializations of apes strongly supports this suggestion.     

The Development and Behaviour of Mycelial Strands in Merulius lacrymans (Wulf.) Fr.: II. Hyphal Behaviour during Strand Formation

The system of hyphal branching by Merulius lacrymans was observedin mycelium which had grown from a wood food-base on to glassslides during incubation in sterile moist chambers. A hierarchyof branches and sub-branches arose from the region of clampconnexions, or nodes, of relatively wide main hyphae. Therewas evidence that the sequence of branches occurring at nodesin basipetal succession represented the time sequence of branchdevelopment at any one node. Later-formed branches at any nodewere smaller than earlier branches, but such earlier branchesusually became smaller towards the tip as growth continued.Mycelial strands were built up by growth and branching of thigmo-tropicallysensitive ‘tendril’ hyphae in association with thewide main hyphae. Tendril hyphae were characteristically narrow,thin-walled hyphae arising both as later-formed branches fromthe nodes of the main hyphae and as the narrowed tips of earlierbranches. Although this branching behaviour could be seen amongstaerial hyphae growing over agar media, hyphae growing in contactwith or within the agar behaved differently and did not formstrands.     

Viviparity: The Maternal-Fetal Relationship in Fishes

SYNOPSIS. Viviparity in the vertebrate line first makes itsevolutionary appearance among fishes. It has independently evolvedin a number of divergent piscine lineages. The 54 families ofextant fishes that bear living young include 40 families ofchondrichthyans (sharks and rays), one montypic family of coelacanths(Latimeria), and 13 families of teleosts. There is fossil evidencefor viviparity in holocephalans and chondrosteans. Viviparitypredominates among sharks and rays (40 families, 99 genera,420 species) but is less widespread among teleosts (13 families,122 genera, 510 species). Following an historical introduction,the organization of the female reproductive system, sites ofgestation, developmental sequences and superfetation are considered.The evolution of viviparity establishes specialized maternal-fetalrelationships, viz., 1) developmental, 2) morphological, 3)trophic, 4) osmoregulatory, 5) respiratory, 6) endocrinological,and 7) immunological. While the latter four categories are brieflynoted the major emphasis is on the trophic relationship andits morphological and developmental basis. First, a generaloverview is presented and then the maternal-fetal trophic relationshipsin each of the major groups of fishes are systematically reviewed.Pertinent anatomical, histological, ultrastructural, developmental,physiological, and biochemical studies are considered. Viviparousfishes are either lecithotrophic, i.e., exclusively yolk dependent,or matrotrophic, i.e., in receipt of a continuous supply ofmaternal nutrients during gestation. Nutrient transfer is accomplishedby 1) oophagy and adelphophagy, 2) placental analogues, and3) the yolk sac placenta. Placental analogues include: externalepithelial absorptive surfaces, e.g., skin, fins, gills; trophonemata,modifications of the uterine epithelia for the secretion ofhistotrophe or "uterine milk"; branchial placentae, close appositionbetween gill epithelia and either uterine or ovarian epithelialvilli; the yolk sac; pericardial amnion and chorion; follicularpseudoplacenta, close apposition between follicle cells andembryonic absorptive epithelia; hypertrophied gut; and trophotaeniae,external rosette or ribbon-like projections of the embryonicgut. Among chondrichthyans, the yolk sac placenta (840–1,050%),trophonematal secretion and embryonic absorbtion of histotrophe(1,680–4,900%) and oophagy and adelphophagy (1.2 x 106%)are the most efficient methods of nutrient transfer. Among teleosts,the follicular pseudoplacenta (1,800–3,900%), trophotaeniae(8,400%) and absorption of ovarian histotrophe through surfaceepithelia and a hypertrophied gut (1,100–34,000%) arethe most efficient. These values stand in contrast to the 30%40%loss of dry weight characteristic of oviparous fishes and viviparouslecithotrophes.     

Symbiosis and Organismal Boundaries

The functional boundaries of organisms may not be exclusiveor discrete. The genomes of organisms include not only stablegenes which replicate faithfully at each division but also transposableelements (jumping genes) and viruses which allow both dynamicvariability within a genome and interspecific genetic interactions.All organisms (with the exception of specially maintained laboratoryorganisms) are in association with other organisms. In closeheterospecific associations (symbioses) there are selectiveadvantages for mechanisms which enable organisms to coordinatetheir activities. In very close, efficient symbioses, mechanismsto share the genetic control of essential, mutually used structures,may have evolved via the activities of movable genetic elements(e.g., transposable elements). Interactions across species boundarieswill include examples on the genetic level, cellular level,and organismal level.     

Mechanisms of Central Die-back of Reynoutria japonica in the Volcanic Desert on Mt. Fuji. A Stochastic Model Analysis of Rhizome Growth

Reynoutria japonica (Polygonaceae) is a pioneer clonal herbcolonising the volcanic desert on Mt. Fuji (height 3776 m),central Japan; establishment of secondary successional speciesoccurs only in the central die-back parts of the clonal standsofR. japonica. Clonal stands were excavated and the morphologyand growth pattern of the rhizomes were investigated with specialreference to the mechanisms of central die-back. Four morphologicalparameters, length of mother rhizomes, and number, branchingpositions and branching angle of daughter rhizomes on respectivemother rhizomes were measured or counted, and their roles inrhizome growth were examined employing a stochastic computersimulation model of the whole stand development. The simulationsclarified that, of these four parameters, the branching anglewas the most influential determinant of the whole pattern ofshoot distribution and that the central die-back was producedwhen the mean branching angle was 40° or smaller. Theseresults strongly infer that the onset of central die-back iscaused intrinsically byR. japonica itself by having the meanbranching angle of 40°. The adaptive significance of thegrowth pattern is discussed in relation to resource acquiringand habitat exploitation strategies of this species. Below-ground morphology; branching angle; central die-back; clonal plant; computer simulation; Japanese knotweed;Reynoutria japonica Houttuyn; rhizome growth pattern     

Hypophysiotropic Centers in the Brain of Amphibians and Fish

The subject is the localization of three different hypophysiotropiccenters in the brain of amphibians and fish. The thyrotropic hormone-releasing hormone (TRH) in Xenapus mayoriginate from the dorsal magno-cellular neurons of the preopticnucleus. This hypothesis is based on correlative changes betweenthese cells and alterations in thyroid activity during metamorphosis.Experimental data are in support of a functional relationshipbetween certain preoptic neurons and the thyrotropic activityof the pituitary. The MSH inhibiting activity of the hypothalamus is effectedby means of an aminergic innervation of the pars intermediain amphibians, teleosts and elasmobranchs. In amphibians theaminergic fibers originate from the caudal part of the paraventricularorgan (PVO); in elasmobranchs probably from the nucleus mediushypothalamicus(NMI); in teleosts the origin still has to beinvestigated. Two centers producing gonadotropic hormone-releasing hormone(GRH) have been demonstrated. Lesion experiments lead to thehypothesis that GRH is produced in the caudal hypothalamus,i.e., in the nucleus infundibularis ventralis of amphibiansand in the nucleus lateralis tuberis of fishes. ImmunoHuorescencestudies indicate in both groups the presence of neurons, infront of the preoptic area in the telencephalon, and these neuronsare immuno-reactive with anti-mammalian LH-RH.     

Branched fibers in dystrophic mdx muscle are associated with a loss of force following lengthening contractions

We demonstrated that the susceptibility of skeletal muscle to injury from lengthening contractions in the dystrophin-deficient mdx mouse is directly linked with the extent of fiber branching within the muscles and that both parameters increase as the mdx animal ages. We subjected isolated extensor digitorum longus muscles to a lengthening contraction protocol of 15% strain and measured the resulting drop in force production (force deficit). We also examined the morphology of individual muscle fibers. In mdx mice 1–2 mo of age, 17% of muscle fibers were branched, and the force deficit of 7% was not significantly different from that of age-matched littermate controls. In mdx mice 6–7 mo of age, 89% of muscle fibers were branched, and the force deficit of 58% was significantly higher than the 25% force deficit of age-matched littermate controls. These data demonstrated an association between the extent of branching and the greater vulnerability to contraction-induced injury in the older fast-twitch dystrophic muscle. Our findings demonstrate that fiber branching may play a role in the pathogenesis of muscular dystrophy in mdx mice, and this could affect the interpretation of previous studies involving lengthening contractions in this animal. skeletal muscle; mdx mouse; lengthening contraction; Duchenne muscular dystrophy     

Pattern of Shoot Growth in Zizyphus mauritiana and Z. oenoplia

Shoot polymorphism and patterns of sylleptic branching and shoottip abscission in two Zizyphus species are reported. Z. mauritiana(ber) produced three types of proleptic shoots: vigorous, normaland spur-type, by reiteration or following pruning;Z. oenopliahad only one type of shoot. Both species revealed up to thirdorder sylleptic branching on the vigorous proleptic shoots witha characteristic pattern and rhythm. Bud dormancy breaking chemicalsgiven as pre-pruning foliar sprays in ber, did not alter thepattern of sylleptic branching. Zizyphus oenoplia produced morefirst and second order sylleptic branches compared to Z. mauritiana.All main axes and 97% of first order sylleptic branches ofZ.mauritiana abscised their apical buds during summer, but inZ. oenoplia all main axes and 54% of first order sylleptic branchesremained active. Shoot tip abscission was almost complete onthe higher order sylleptic branches of both species. Activefirst order branches in Z. oenoplia were confined to the tophalf of the shoot. Both species had a few dormant apical budsduring the summer on their sylleptic branches, emerging mostlyfrom the middle portion of the shoot. Some of the most vigorousfirst order branches of Z. oenoplia, which had dormant apicalbuds during summer, showed a change in the frequency of syllepticbranching when they resumed growth following monsoon showers.These characteristic growth and branching patterns may haveadaptive value for canopy development under arid and semi-aridconditions. Copyright 1999 Annals of Botany Company Ber, branching pattern, shoot polymorphism, shoot tip abscission, sylleptic branching, Zizyphus mauritiana, Zizyphus oenoplia.     

Lactate Dehydrogenase Isozymes, Gytochrome Oxidase Activity, and Muscle Ions of the Rattail (Coryphaenoides sp.)

SYNOPSIS. Investigations into the behavior of molecules in organismsoccupying unique environments may provide a better insight intothe functions of these same molecules in organisms from morecommon habitats. An organism well suited for such analyses isthe rattail (Coryphaenoides sp.), a deep-sea teleost. The photoreceptorcells of the retina are predominantly rods. Although the lactatedehydrogenase A₁, and B₄, isozymes are present in this fish,the E₄ isozyme (found in the retina of many teleosts) is absentin the rattail retina. The rattails possess a lower cytochromeoxidase activity than shallow water fish. The sodium concentrationis higher, and the potassium concentration lower in the rattailas compared with surface marine fish. The patterns of molecularsynthesis and concentrations in the rattail may be related tosuch factors as light intensity, hydrostatic pressure, and temperaturewhich exists in the deep-sea environment.     

Two Geometrical Models of Branching of Botanical Trees

A botanical tree may be regarded as a system of axes which developby repeated bifurcation. A complicated tree can be describedby a few parameters of bifurcation which determine the geometryof the bifurcation process. A bifurcation model (H-model) haspreviously been proposed and shown to be useful for comparisonwith trees which have branch complexes which approach a horizontalplane (e.g. Terminalia). Another bifurcation model (P-model)is now proposed which is appropriate to branching systems inwhich successive branch planes are perpendicular to each other(e.g. Cameraria and Tabernaemontana of the Apocynaceae). Bymodification of the P-model to take into account a geotropiceffect, a more realistic branching model for one kind of treespecies is produced. The relationship among these geometricalmodels of branching is discussed and illustrated with computersimulations. computer simulations, tree crown geometry, branching patterns, bifurcation models, Terminalia, Cameraria latifolia, Tabernaemontana, sp     

Factors Affecting Clonal Diversity and Coexistence

Recent genetic studies of asexually reproducing fishes in thegenus Poeciliopsis (Poeciliidae) revealed abundant variationin the form of multiple sympatric clones. Recurrent hybridizationsbetween sexual species provides the principal source of clonalvariation. The hybrids are spontaneously endowed with a clonalreproductive mechanism that perpetuates a high level of heterozygosity.Migration within and between river systems, and mutations, alsocontribute to clonal diversity in these fish. Coexistence amongdifferent clones and with the sexual ancestors depends in partupon specializations characteristic of individual clones. Clonalreproduction is an efficient mechanism for freezing a portionof the niche-width variation contained in the gene pool of themore broadly adapted, sexual ancestors. Multiclonal populationsachieve significantly higher densities relative to the sexualforms than do monoclonal populations. This relationship is afunction of the clonal variability upon which natural selectioncan act and upon the capacity of a multiclonal population tobetter exploit a heterogeneous environment through niche diversification.In all-female organisms such as Poeciliopsis, which are dependentupon sexual species for insemination, competitive abilitiesprobably are at a premium in the densely populated pools andarroyos of the Sonoran Desert. Competitive abilities are probablyless important for truly parthenogenetic clones which rely oncolonization abilities to escape from their sexual ancestorsand from other clones.     

Divergent ontogenies of trophic morphology in two closely related haplochromine cichlids

Fish develop morphological specializations in their trophic and locomotor systems as a result of varying functional demands in response to environmental pressures at different life stages. These specializations should maximize particular performances in specialists, adapting them to their trophic and habitat niches at each ontogenetic stage. Because differential growth rates of the structural components comprised in the head are likely to be linked to the diet of a fish throughout its development, we investigated the ontogenetic development of two haplochromine cichlid species belonging to different trophic guilds. We employed geometric morphometric techniques to evaluate whether starting from morphologically similar fry they diverge into phenotypes that characterize trophic guilds and locomotor types. Our examination of overall body shape shows that certain specialized morphological features are already present in fry, whereas other traits diverge through ontogeny due to differences in species‐specific allometric variation. Allometric shape variation was found to be more relevant for the biter specialist than for the sucker morphotype. Our results confirm that phenotypic changes during ontogeny can be linked to dietary and habitat shifts in these fish. Furthermore, evidence for an integrated development of trophic and locomotor specializations in morphology was observed. J. Morphol. 276:860–871, 2015. © 2015 Wiley Periodicals, Inc.     

Using a nonrecursive formula to determine cladogram probabilities

Three properties of bifurcating branching diagrams that are used for representing a specific number of taxa are (1) the number of possible arrangements, (2) the number of possible topologies, and (3) the probabilities of formation according to particular models of cladogenesis. Of these, the probabilities have received the least attention in the literature. Indeed, many biologists would be astonished by the observation that the probability of a commonly cited cladogram containing 35 phyla of the animal kingdom is < 0.0072% of the value of the average probability taken over all possible cladograms! We reviewed works on cladogram arrangements and topologies and developed a computer-generated table of enumerations that extends and corrects such tables in the literature. We also developed a nonrecursive formula for the determination of cladogram probabilities. This formula facilitates calculation and thereby should promote use of cladogram probabilities, which might provide more accurate null hypotheses for tests of cladogenic events than do considerations of cladogram arrangements or topologies.     

Narrow-leafed Lupins with Restricted Branching

Crop phenology is one of the most important characters influencingproductivity in a given environment. Narrow-leafed lupin (Lupinusangustifolius L.) is a major grain legume crop in southern Australiawith general phenological adaptation to this Mediterranean-typeenvironment. However, it is an indeterminate crop with severalassociated limitations to productivity, such as overlappingvegetative and reproductive growth, late grain filling and sometimesexcessive vegetative growth. Here we studied two novel typesof narrow-leafed lupin with restricted branching, which mightbe useful for overcoming these problems. These restricted branchinglupins arose spontaneously within a breeding population, inthe case of ‘Tallerack’, and within a farmer's cropin the case of ‘ Hurst’ and we compared them withthe ‘Merrit’, which is widely grown and has thenormal indeterminate branching habit. The morphology and developmentof the main shoot of these genotypes were similar. However,‘Hurst’ had much larger leaves. There were alsostriking differences in the lateral branches of the restrictedbranching types; they had fewer leaves than ‘Merrit’and flowered earlier. These differences were most marked in‘ Hurst’, where the upper main stem branches werereduced to a single floret in the axil of main stem leaves,and these flowers often exhibited abnormal morphology. Copyright2000 Annals of Botany Company Lupinus angustifolius L., narrow-leafed lupin, adaptation, development, morphology, branching, leaves, mutant, plastochron, phyllochron, floral initiation, flowering.     

Branching Principles Governing the Architecture of Cornus kousa (Cornaceae)

The complex structure of the crown of Cornus kousa, generallyfive-forked in vegetative branching and two-forked in reproductivebranching, is analysed quantitatively and described by two basicbranching principles: decussate phyllotaxy and the resettingrule for planes of branching. Most Cornus species have opposite,decussate phyllotaxis. The leaf pair (with axillary buds) definesthe branching plane of a node. Because of regular phyllotaxis,the fundamental branching pattern is that every branching planealong an axis is perpendicular to the preceding one. However,the first node of a lateral horizontal shoot always has a horizontalbranching plane; we term this the resetting rule. We observedthat resetting occurs when the first nodes initiated in thevertical plane are repositioned by a twisting of their firstinternodes. All later nodes alternate directions, i.e. showusual decussate alternation. Foliage leaf nodes usually producethree-forked branchings. When vegetative winter buds are formed,a foliar node and adjacent scale leaf node produce a five-forkedbranching. When reproductive winter buds with a terminal inflorescenceare formed, the last foliar node and two adjacent scale leafnodes can produce a variety of branchings but usually producean equal two-forked branching. To understand better the architecturein C. kousa, we contrast it with C. capitata which does notproduce buds with scale leaves and whose vegetative nodes areclearly separated. Copyright 1999 Annals of Botany Company Branching pattern, Cornaceae, Cornus kousa, decussate branching, dogwood, Japanese strawberry tree, tree architecture, tree geometry.     

UV-B-induced damage and photoreactivation in three species of Boeckella (Copepoda, Calanoida)

Solar ultraviolet (UV) radiation poses a threat to most livingorganisms. Aquatic organisms have evolved three basic mechanismsto cope with harmful levels of radiation. Two mechanisms, avoidance(e.g. vertical migration) and photoprotection (e.g. productionof photoprotective compounds that act as filters, antioxidants,etc.), serve to minimize the dose of UV radiation that reachesthe organism's vital structures (DNA, membranes, etc.). Thethird mechanism, repair (e.g. dark repair mechanisms, such asnucleotide excision repair; or photoreactivation mechanisms,such as photoenzymatic repair), serves to repair the damagefollowing UV exposure. Here, we compare the vulnerability toUV-B radiation of three copepod species (Boeckella brevicaudata,Boeckella gibbosa, and Boeckella gracilipes) that occur in lakesthat differ in UV-B penetration and depth. Our aim was to gaininsight into the significance of each of the three mechanismsin different UV-B environments. Results from a 3-day ‘insitu’ incubation in ultra-oligotrophic Lake Toncek showedthat B.gracilipes is highly vulnerable to UV-B and UV-A radiation.In contrast, virtually no mortality was observed in B.gibbosaand B.brevicaudata during the same period. In order to discriminatethe contribution of photoprotection and photoreactivation, thethree species were subsequently exposed in the laboratory toan artificial source of UV-B radiation, both in the presenceand absence of visible radiation (recovery radiation). The photoprotectionpotential (i.e. resistance to UV-B in the absence of recoveryradiation) of B.gracilipes and B.gibbosa was lower than thatof B.brevicaudata. On the other hand, photoreactivation (higherresistance to UV-B in the presence of recovery radiation) wasobserved in B.brevicaudata and B.gibbosa, but not in B.gracilipes.To cope with damaging UV-B levels in nature, B.gracilipes dependsexclusively on the attenuation by the external media (i.e. avoidance).Although B.gibbosa tends to avoid the surface waters of lakes,it also occurs in shallow transparent pools. Most likely itsability to survive in these shallow, high UV environments isdue to its photoreactivation potential. Finally, despite itsoccurrence in highly turbid lakes, B.brevicaudata seems extremelywell suited to cope with UV-B radiation thanks to a combinationof photoreactivation and photoprotection.     

The Hexagonal Branching Pattern of Rhizomes of Alpinia speciosa L. (Zingiberaceae)

Alpinia speciosa L. (Zingiberaceae) is a striking example ofa plant in which an organized architectural structure fulfilsa basic functional requirement - the economical explorationand exploitation of the substrate. This rhizomatous plant spreadslaterally in a predictable manner by vegetative growth to forma clone of separated ramets. A morphological study establishes‘rules of growth’ governing the success and failureof lateral branches (sympodial units), their lengths, and theirrelative angles. The basic branching pattern of this rhizome system approximatesto that of a hexagonal grid - the most economical structuralshape that uniformly ‘samples’ a plane surface.It is shown that the significant deviation of branching anglefound in Alpinia from that required to develop an exact hexagonalnetwork will virtually eliminate aerial shoot ‘clashes’in which the placing of one aerial shoot coincides with thatof another aerial shoot, whilst at the same time enhancing thebenefits of the hexagonal system. The broader implications ofthe organised architecture of plants are emphasised. rhizome, branching pattern, architecture, productivity     

Enolase from Trypanosoma brucei, from the amitochondriate protist Mastigamoeba balamuthi, and from the chloroplast and cytosol of Euglena gracilis: pieces in the evolutionary puzzle of the eukaryotic glycolytic pathway

Genomic or cDNA clones for the glycolytic enzyme enolase wereisolated from the amitochondriate pelobiont Mastigamoeba balamuthi,from the kinetoplastid Trypanosoma brucei, and from the euglenidEuglena gracilis. Clones for the cytosolic enzyme were foundin all three organisms, whereas Euglena was found to also expressmRNA for a second isoenzyme that possesses a putative N-terminalplastid-targeting peptide and is probably targeted to the chloroplast.Database searching revealed that Arabidopsis also possessesa second enolase gene that encodes an N-terminal extension andis likely targeted to the chloroplast. A phylogeny of enolaseamino acid sequences from 6 archaebacteria, 24 eubacteria, and32 eukaryotes showed that the Mastigamoeba enolase tended tobranch with its homologs from Trypanosoma and from the amitochondriateprotist Entamoeba histolytica. The compartment-specific isoenzymesin Euglena arose through a gene duplication independent of thatwhich gave rise to the compartment-specific isoenzymes in Arabidopsis,as evidenced by the finding that the Euglena enolases are moresimilar to the homolog from the eubacterium Treponema pallidumthan they are to homologs from any other organism sampled. Inmarked contrast to all other glycolytic enzymes studied to date,enolases from all eukaryotes surveyed here (except Euglena)are not markedly more similar to eubacterial than to archaebacterialhomologs. An intriguing indel shared by enolase from eukaryotes,from the archaebacterium Methanococcus jannaschii, and fromthe eubacterium Campylobacter jejuni maps to the surface ofthe three-dimensional structure of the enzyme and appears tohave occurred at the same position in parallel in independentlineages.     

Reflections on the Evolution of Piscine Viviparity

Viviparity first makes its evolutionary appearance within thecraniate-vertebrate line among fishes. We estimate that it hasindependently evolved at least 42 times in five of the ninemajor groups of fishes. Viviparity is the dominant mode of reproductionamong the cartilaginous sharks and rays, i.e., 55% of approximately900 living species. It is less prevalent among the five majorgroups of bony fishes, i.e., 2–3% of an estimated 20,000or more species. The evolution of viviparity from oviparityinvolves: 1) a shift from external to internal fertilization;2) retention of embryos in the female reproductive system; 3)utilization of the ovaryor oviduct as sites of gestation; 4)structural and functional modification of the embryo and thefemale reproductive system and; 5) modification of extant endocrinemechanisms controlling reproduction. Viviparity offers selectiveadvantages to parents and offspring, such as: 1) enhanced survivalof offspring; 2) compensation for low fecundity; 3) amplificationof reproductive niches to reduce competition; 4) exploitationof pelagic niches; 5) colonization of new habitats; and 6) increasedenergetic efficiency in viviparous matrotrophes. Its principaldisadvantages include: 1) reduced fecundity; 2) cost to thefemale; and 3) risk of brood loss through maternal death. Acquisitionof viviparityestablishes new maternal-embryonic relationships,namely: 1) trophic; 2) osmoregulatory and excretory; 3) respiratory;4) endocrinological; and 5) immunological. In sharks, rays,and the coelacanth, gestation takes place in the oviduct, butin teleosts gestation occurs either in the ovarian follicleor ovarian lumen. The cystovarian teleostean ovary is hypothesizedto function both as ovary and oviduct. Oviductal, ovarian lumenal,andfollicular epithelial cells are the maternal sites of metabolicexchange. Metabolic exchange inembryos takes place across theepithelia of the general body surface and its derivatives oracross the gut epithelium and its derivatives. Four patternsof piscine placentation have evolved,namely: 1) yolk sac; 2)follicular; 3) branchial; and 4) trophotaenial placentae. Thepericardial amniochorion, the embryonic portion of the follicularplacenta, occurs in poeciliids and several other teleosteangroups. Developmental, it is nearly identical to the anterioraminochorionic fold of tetrapod vertebrates. Trophotaeniae areexternal rosette or ribbon-like structuresthat have evolvedin four orders of teleosts by heterochrony, i.e., acceleratedoutgrowth and differentiation of the embryonic hind gut. Withthe possible exception of the coelacanth, theyolk sac placentaoccurs only in sharks. We estimate that it has independentlyevolved between 11 and 20 times. It displays considerable diversity.Evolution of the yolk sac placenta entails retention of theyolk sac and secondary differentiation of its distal portionfor implantation and maternal tissue-embryonic tissue metabolicexchange and its proximal portion for oviductal fluid-embryonictissue exchange. The yolk stalk lengthens, is modified intoan umbilical stalk, and establishes a site of autotomy at theembryo-umbilical stalk junction. The lumenal wall of the oviductbecomes competent to function as a site of implantation.