Ancestral Features (ancestral + feature)

Distribution by Scientific Domains

Selected Abstracts

Expression of Hoxa-11 and Hoxa-13 in the pectoral fin of a basal ray-finned fish, Polyodon spathula: implications for the origin of tetrapod limbs

Brian D. Metscher
Summary Paleontological and anatomical evidence suggests that the autopodium (hand or foot) is a novel feature that distinguishes limbs from fins, while the upper and lower limb (stylopod and zeugopod) are homologous to parts of the sarcopterygian paired fins. In tetrapod limb development Hoxa-11 plays a key role in differentiating the lower limb and Hoxa-13 plays a key role in differentiating the autopodium. It is thus important to determine the ancestral functions of these genes in order to understand the developmental genetic changes that led to the origin of the tetrapod autopodium. In particular it is important to understand which features of gene expression are derived in tetrapods and which are ancestral in bony fishes. To address these questions we cloned and sequenced the Hoxa-11 and Hoxa-13 genes from the North American paddlefish, Polyodon spathula, a basal ray-finned fish that has a pectoral fin morphology resembling that of primitive bony fishes ancestral to the tetrapod lineage. Sequence analysis of these genes shows that they are not orthologous to the duplicated zebrafish and fugu genes. This implies that the paddlefish has not duplicated its HoxA cluster, unlike zebrafish and fugu. The expression of Hoxa-11 and Hoxa-13 in the pectoral fins shows two main phases: an early phase in which Hoxa-11 is expressed proximally and Hoxa-13 is expressed distally, and a later phase in which Hoxa-11 and Hoxa-13 broadly overlap in the distal mesenchyme of the fin bud but are absent in the proximal fin bud. Hence the distal polarity of Hoxa-13 expression seen in tetrapods is likely to be an ancestral feature of paired appendage development. The main difference in HoxA gene expression between fin and limb development is that in tetrapods (with the exception of newts) Hoxa-11 expression is suppressed by Hoxa-13 in the distal limb bud mesenchyme. There is, however, a short period of limb bud development where Hoxa-11 and Hoxa-13 overlap similarly to the late expression seen in zebrafish and paddlefish. We conclude that the early expression pattern in tetrapods is similar to that seen in late fin development and that the local exclusion by Hoxa-13 of Hoxa-11 from the distal limb bud is a derived feature of limb developmental regulation. [source]

Tornaria of hemichordates and other dipleurula-type larvae: a comparison,

L. P. Nezlin
The evolutionary origin of phylum Chordata is the subject of intensive discussion, with the most conflicting views prevalent. One popular theory advocates the separation of chordates from a dipleurula-like ancestor. Thus the dipleurula-type larvae (tornaria of enteropneusts, auricularia and bipinnaria of echinoderms) are considered to recapitulate the ancestral features and the direct evolutionary path from Echinodermata and Hemichordata to Chordata (i.e. Garstang 1894 Zool. Anzeiger 27, 122,125; Grobben 1908 Verh. Zool.-Bot. Ges. Wien 58, 491,511; Dillon 1965 Evolution 19, 436,446; Jollie 1973 Acta Zool. (Stockholm) 54, 81,100; Ivanova-Kazas and Ivanov 1987 Sov. J. Mar. Biol. 13, 67,80; Crowther and Whittaker 1992 J. Neurophysiol. 23, 280,292; Lacalli 1994 Am. Zool. 34, 533,541; Lacalli et al. 1999 Proc. R. Soc. Biol. Series B 266, 1461,1470; Nielsen 1999 Dev. Genes Evol. 209, 198,205). Comparison of the nervous system in enteropneust tornariae and echinoderm larvae has revealed however, striking differences in distribution of biogenic amines and cholinesterase activity. In tornariae, monoamine-containing cells concentrate in the aboral and oesophageal ganglia. In echinoderms, they are located along the ciliary bands throughout their length. The difference in distribution of cholinesterase activity in each group reasonably suggests that acetylcholine-dependent control of locomotion also differs. Our data do not support the homology of the dipleurula-type larvae. Therefore we believe in the course of adaptive evolution, larvae of certain marine invertebrates acquired a set of common morphological and behavioural characteristics, yet retained different physiological mechanisms of behavioural regulation. Thus, similarities in the dipleurula-type larvae (tornaria, auricularia or bipinnaria, and actinotrocha) may have originated from convergence rather then from a common dipleurula-type predecessor. In consequence we must call into question any attempt to trace the ancestors of Chordata to the dipleurula-type animal. [source]

Variation and causal factors of craniofacial robusticity in Patagonian hunter-gatherers from the late Holocene

Valeria Bernal
Fueguian-Patagonian skulls have been characterized as some of the most robust of any modern crania. However, the causal factors of such robusticity remain unsettled. We assess within- and among-sample cranial robusticity of seven samples from continental Patagonia and Tierra del Fuego, using geometric morphometric techniques. In addition, the biomechanical, phylogenetic, and climatic hypotheses proposed to account for robusticity in such samples are discussed. Two Amerindian samples of farmers and two early middle Holocene samples from South America were included. The results show: 1) large variation in craniofacial robusticity among Patagonian samples, with the highest robusticity in samples from south continental Patagonia and Isla Grande of Tierra del Fuego, whereas central and north Patagonian samples display the same degree of robusticity as farmer samples; 2) that early middle Holocene samples display lower levels of robusticity than South Patagonian samples; and 3) strong association between latitude and craniofacial robusticity, with the most robust craniofacial morphologies occurring at the highest latitudes. In consequence, neither masticatory stress nor retention of ancestral features is supported by the morphological evidence analyzed. Hence it is hypothesized that endocrine changes related to cold climate may be a plausible explanation for several craniofacial features found in Fueguian and south continental Patagonian samples, such as their large masticatory component, and pronounced supraorbital ridge and glabellar region. Am. J. Hum. Biol. 18:748,765, 2006. 2006 Wiley-Liss, Inc. [source]

Xylem heterochrony: an unappreciated key to angiosperm origin and diversifications

All angiosperms can be arranged along a spectrum from a preponderance of juvenile traits (cambial activity lost) to one of nearly all adult characters (cambium maximally active, mature patterns realized rapidly early in ontogeny). Angiosperms are unique among seed plants in the width of this spectrum. Xylem patterns are considered here to be indicative of contemporary function, not relictual. Nevertheless, most families of early-divergent angiosperms exhibit paedomorphic xylem structure, a circumstance that is most plausibly explained by the concept that early angiosperms had sympodial growth forms featuring limited accumulation of secondary xylem. Sympodial habits have been retained in various ways not only in early-divergent angiosperms, but also among eudicots in Ranunculales. The early angiosperm vessel, relatively marginal in conductive abilities, was improved in various ways, with concurrent redesign of parenchyma and fibre systems to enhance conductive, storage and mechanical capabilities. Flexibility in degree of cambial activity and kinds of juvenile/adult expressions has been basic to diversification in eudicots as a whole. Sympodial growth that lacks cambium, such as in monocots, provides advantages by various features, such as organographic compartmentalization of tracheid and vessel types. Woody monopodial eudicots were able to diversify as a result of production of new solutions to embolism prevention and conductive efficiency, particularly in vessel design, but also in parenchyma histology. Criteria for paedomorphosis in wood include slow decrease in length of fusiform cambial initials, predominance of procumbent ray cells and lesser degrees of cambial activity. Retention of ancestral features in primary xylem (the ,refugium' effect) is, in effect, a sort of inverse evidence of acceleration of adult patterns in later formed xylem. Xylem heterochrony is analysed not only for all key groups of angiosperms (including monocots), but also for different growth forms, such as lianas, annuals, various types of perennials, rosette trees and stem succulents. Xylary phenomena that potentially could be confused with heterochrony are discussed. Heterochronous xylem features seem at least as important as other often cited factors (pollination biology) because various degrees of paedomorphic xylem are found in so many growth forms that relate in xylary terms to ecological sites. Xylem heterochrony can probably be accessed during evolution by relatively simple gene changes in a wide range of angiosperms and thus represents a current as well as a past source of variation upon which diversification was based. Results discussed here are compatible with both current molecular-based phylogenetic analyses and all recent physiological work on conduction in xylem and thus represent an integration of these fields. 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 161, 26,65. [source]