Focal Review: The Origin(s) of Modern AmphibiansAnderson, Jason
doi: 10.1007/s11692-008-9044-5pmid: N/A
The recent description of the stem batrachian Gerobatrachus has changed the terms of the ongoing debate on the origin of extant amphibians (Lissamphibia: frogs, salamanders, and the limbless caecilians). This important fossil, through a shared mosaic of unique derived salientian and urodele characters, links frogs and salamanders with an archaic group of fossil amphibians known as amphibamid temnospondyls. The present paper reviews the impact of this fossil on morphological and molecular phylogenies, and divergence timing estimates based on molecular models and the fossil record. In morphology, most recent efforts have focused on better characterizing the anatomy and relationships of amphibamid temnospondyls. Progress has also been made with the complete description of the earliest caecilian Eocaecilia; however, the question of caecilian origins remains unresolved at present. The large scale phylogenetic analyses all agree on the overall tetrapod tree phylogenetic structure, and the largest analyses agree that the origin of at least frogs and salamanders among fossils from family Amphibamidae. Conversely, all molecular based analyses find a monophyletic Lissamphibia, and a Batrachia terminal dichotomy, which raises questions over either the validity of morphological analyses that support lissamphibian polyphyly or about the possibility of long branch attraction given the short internal divergences and long subsequent branches. Paradoxically, the estimated date of the lissamphibian divergence best matches the fossil record if timed to the split between lepospondyls and temnospondyls. Future research should focus on development and fine details of cranial anatomy of fossil and extant amphibians to produce new evidence and clarity into the question of lissamphibian, and especially caecilian, origins.
Evolutionary Origins of the Neural Crest and Neural Crest CellsHall, Brian
doi: 10.1007/s11692-008-9033-8pmid: N/A
I evaluate the lines of evidence—cell types, genes, gene pathways, fossils—in putative chordate ancestors—cephalochordates and ascidians—pertaining to the evolutionary origin of the vertebrate neural crest. Given the intimate relationship between the neural crest and the dorsal nervous system during development, I discuss the dorsal nervous system in living (extant) members of the two groups, especially the nature, and genes, and gene regulatory networks of the brain to determine whether any cellular and/or molecular precursors (latent homologues) of the neural may have been present in ancestral cephalochordates or urochordates. I then examine those fossils that have been interpreted as basal chordates or cephalochordates to determine whether they shed any light on the origins of neural crest cell (NCC) derivatives. Do they have, for example, elements of a head skeleton or pharyngeal arches, two fundamental vertebrate characters (synapomorphies)? The third topic recognizes that the origin of the neural crest in the first vertebrates accompanied the evolution of a brain, a muscular pharynx, and paired sensory organs. In a paradigm-breaking hypothesis—often known as the ‘new head hypothesis’—Carl Gans and Glen Northcutt linked these evolutionary innovations to the evolution of the neural crest and ectodermal placodes (Gans and Northcutt Science 220:268-274, 1983. doi:
10.1126/science.220.4594.268
; Northcutt and Gans The Quarterly Review of Biology 58:1–28, 1983. doi:
10.1086/413055
). I outline the rationale behind the new head hypothesis before turning to an examination of the pivotal role played by NCCs in the evolution of pharyngeal arches, in the context of the craniofacial skeleton. Integrations between the evolving vertebrate brain, muscular pharynx and paired sensory organs may have necessitated that the pharyngeal arch skeletal system—and subsequently, the skeleton of the jaws and much of the skull (the first vertebrates being jawless)—evolved from NCCs whose developmental connections were to neural ectoderm and neurons rather than to mesoderm and connective tissue; mesoderm produces much of the vertebrate skeleton, including virtually all the skeleton outside the head. The origination of the pharyngeal arch skeleton raises the issue of the group of organisms in which and how cartilage arose as a skeletal tissue. Did cartilage arise in the basal proto-vertebrate from a single germ layer, cell layer or tissue, or were cells and/or genes co-opted from several layers or tissues? Two recent studies utilizing comparative genomics, bioinformatics, molecular fingerprinting, genetic labeling/cell selection, and GeneChip Microarray technologies are introduced as powerful ways to approach the questions that are central to this review.
Modifier Selection by Transgenes: The Case of Growth Hormone Transgenesis and Hyperactive Circling MiceChaudhry, A.; Marsh-Rollo, S.; Aksenov, V.; Rollo, C.; Szechtman, H.
doi: 10.1007/s11692-008-9036-5pmid: N/A
Deleterious impacts of major mutations can be ameliorated by stabilising selection acting on modifier genes. We hypothesise that a new hyperactive circling mouse (counterspin: Cr) arises when modifier genes inadvertently selected to ameliorate the negative impacts of a growth hormone transgenic insertion segregate into the normal genetic background that lacks the transgene. We hypothesise that such modifiers generate a phenotype “mirror image” to the transgenics on the otherwise normal background. We highlight this by testing a priori hypotheses that counterspin and transgenic growth hormone mice deviate oppositely from normal mice across a broad spectrum of characteristics. Results spanning growth, sensorimotor performance, cognition and striatal neurotransmitters provide strong circumstantial evidence for the hypothesis. In a more direct test for selection in the transgenic mice, we found that those examined in 2008 slept ~3 h/d less than they did 14 years ago (P < 0.0005). This is a profound change strongly supporting the reality of modifier selection in these mice. Our results highlight that modifiers may act powerfully on genetically engineered constructs given a genetically variable background. Furthermore, we suggest that modifier selection might provide a novel method for deriving genetic models, and specifically, models phenotypically opposite to engineered constructs or natural mutations.
Primate Growth in the Slow Lane: A Study of Inter-Species Variation in the Growth Constant AMumby, Hannah; Vinicius, Lucio
doi: 10.1007/s11692-008-9040-9pmid: N/A
Primates grow and develop slowly for mammalian standards. Charnov showed that primates grow at only about 40% of the rates observed in other mammals of similar size. However, previous estimates of growth rates in primates were derived from regressions of adult body weight on age at first reproduction in different species, and therefore represent only an average trend for primates. Based on Charnov’s ‘growth law’, we estimated the growth constant A directly from published growth curves for 36 primate species from strepsirrhines to apes. We show that although primate growth is slow in all sampled species in comparison with the mammalian average, there is significant variation around the primate mean. Lemurids are particularly interesting due to their wide range of A values, and further study is required to determine whether environmental unpredictability could lead to the evolution of both very fast and very slow grow in different species. Results also indicate significant negative correlations between the growth constant A and both age at first reproduction and duration of the juvenile period, lending support to the juvenile risk hypothesis.
Mandibular Shape, Ontogeny and Dental Development in Bonobos (Pan paniscus) and Chimpanzees (Pan troglodytes)Boughner, Julia; Dean, M.
doi: 10.1007/s11692-008-9043-6pmid: N/A
The postnatal ontogenetic patterns and processes that underlie species differences in African ape adult mandibular morphology are not well understood and there is ongoing debate about whether African ape faces and mandibles develop via divergent or parallel trajectories of shape change. Using three-dimensional (3D) morphometric data, we first tested when in postnatal development differences in mandibular shape are initially evident between sister species Pan troglodytes and P. paniscus. Next, we tested whether each species has a distinct and non-parallel trajectory of mandibular development. Mandibles sampled across a broad developmental range of wildshot bonobos (n = 44) and chimpanzees (n = 59) were radiographed and aged from their dental development. We then collected 3D landmark surface data from all the mandibles. A geometric morphometric analysis of size-corrected 3D data found that bonobos and chimpanzees had parallel and linear ontogenetic trajectories of mandibular shape change. In contrast, mandibular shape was statistically different between P. paniscus and P. troglodytes as early as infancy, suggesting that species shape differences are already established near or before birth. A linear and stable trajectory of shape change suggests that mandibular ontogeny in these apes is unimpacted by non-linear variation in tooth developmental timing.