How does a salamander transform from an aquatic larvae to a ..
The aquatic salamander appears to have developed smaller forelegs and non-existent hind legs to adapt to its habitat
Cave-dwelling aquatic "dragons" are actually salamanders
Freshwater habitats of coastal plains are refugia for many divergent vertebrate lineages, yet these environments are highly vulnerable to sea-level fluctuations, which suggest that resident communities have endured dynamic histories. Using the fossil record and a multi-locus nuclear phylogeny, we examine divergence times, biogeography, body size evolution and patterns of community assembly of aquatic salamanders from North American coastal plains since the Late Cretaceous. At least five salamander families occurred on the extensive Western Interior Coastal Plain (WICP), which existed from the Late Cretaceous through the Eocene. Four of these families subsequently colonized the emergent Southeastern Coastal Plain (SECP) by the Early Oligocene to Late Miocene. Three families ultimately survived and underwent extensive body size evolution in situ on the SECP. This included at least two major size reversals in recent taxa that are convergent with confamilial WICP ancestors. Dynamics of the coastal plain, major lineage extinctions and frequent extreme changes in body size have resulted in significant shuffling of the size structure of aquatic salamander communities on this shifting refuge since the Cretaceous.
Like the majority of frogs and toads, many salamanders undergo an obligate metamorphosis that allows for the exploitation of both aquatic and terrestrial habitats during ontogeny. However, some salamander species express an alternate developmental mode in which they forego metamorphosis and remain in the aquatic habitat throughout their lifetimes (). Nonmetamorphic forms are termed paedomorphic because they maintain juvenile features of the ancestral condition as they mature reproductively into large, larval forms (). The exemplar of salamander paedomorphosis is the Mexican axolotl (Ambystoma mexicanum). Ambystoma mexicanum (Am) belongs to a group of several closely related species collectively known as the tiger salamander species complex (). Salamanders of this complex occupy a variety of North American breeding habitats ranging from temporary vernal pools to large permanent lakes. Among these habitats, populations are highly variable for metamorphic timing and expression of paedomorphosis. Some populations express metamorphosis (e.g., A. tigrinum tigrinum, Att) or paedomorphosis like Am, while in other populations both phenotypes are observed at varying frequencies. Presumably, the expression of paedomorphosis is an opportunistic strategy that allows individuals to more successfully colonize relatively permanent aquatic niches (; ). Paedomorphic tiger salamanders are found in newly created habitats like cattle watering troughs and wastewater treatment ponds (; ), as well as in stable, large lake systems ().
It lacks hind limbs and has very tiny forelimbs
Despite aforementioned differences, caudate digit development does respond to the patterning protein Sonic hedgehog (SHH) in a manner similar to that observed in other tetrapods. For example, manipulation of SHH expression readily induces sequential digit loss in the axolotl, Ambystoma mexicanum. SHH provides as well a developmental explanation for evolutionary digit loss among closely related species in the scincid lizard genus Hemiergis: changes in digit number (2 fingers/2 toes, 3/3, 4/4, 5/5) correlate strongly with SHH temporal expression . If temporal expression of SHH does specify differences within Hemiergis, then comparable SHH alterations could influence digit number variation in other closely related tetrapod species. Heterochronic changes in SHH expression (and attendant regulatory proteins such as GLI3 ) offer a tenable mechanism for parallel digit loss in dwarf salamanders or, alternatively, digit re-evolution in the Edwards Plateau complex.
We examined the genetic contribution of a major-effect QTL (met) that is strongly associated with the discrete expression of metamorphosis vs. paedomorphosis in interspecific crosses using Att and a laboratory strain of Am (). Previous studies have shown that a dominant allele from Att (metAtt) and a recessive allele from lab Am (metlab) results in metamorphosis in Att/Am hybrids and that metAtt/metlab and metlab/metlab backcross genotypes are strongly associated with metamorphosis and paedomorphosis, respectively (; , ) (). Here we describe a newly identified and highly informative expressed sequence tag marker for met called contig325. This marker is informative in the majority of Ambystoma species (data not shown) and thus represents an important new candidate for studies of developmental timing variation in natural populations. We also describe a large, newly created backcross population using wild-caught Am individuals called WILD2. WILD2 and the smaller WILD1 backcrosses () may differ from lab Am (LAB) backcrosses as a result of differences in the effects of met alleles and/or genetic background effects. To test this idea, we examined contig325 within the context of all available backcross populations (LAB, WILD1, and WILD2) to infer genetic changes that have modified the paedomorphic response of the natural Am population during domestication of the laboratory strain at Indiana University (The Axolotl Colony). Because WILD2 is the largest Att/Am backcross resource ever obtained (N = 457), we were also able to accurately and reliably assess the genetic contribution of met to a second form of phenotypic variation: continuous variation in metamorphic timing. Our results show that met contributes genetically to both discrete and continuous forms of metamorphic timing variation. This result suggests a linkage between the evolutionary maintenance of biphasic life cycles and the evolution of alternate developmental modes.
NOVA - Official Website | Evolution in Action: Salamanders
Novel developmental modes may evolve as a result of genetic changes in developmental timing or heterochrony (; ; ; ; ; ). The paedomorphic developmental mode of the Mexican axolotl (Am) is a classic example of heterochrony. Paedomorphosis in Am presumably evolved as a result of a genetic change that blocked the initiation of metamorphosis in a biphasic ancestor, and this resulted in larval-form adults. In support of this idea, we found that within interspecific crosses using Am and metamorphic Att, the segregation of genotypes at a major-effect QTL (met) was associated with the expression of metamorphosis vs. paedomorphosis. This result supports the long-held idea that paedomorphosis in Am evolved via saltation (; , ; ; ; ; ; ; ). However, we also identified differences in gene effect that have evolved rapidly between the laboratory and wild strains of Am (), and we found that met contributed to a second form of phenotypic variation: continuous variation in age at metamorphosis. This later result indicates that expression of paedomorphosis is associated with genetic changes that alter developmental timing (contra ; ). Below, we review the primary results and then explain how a genetic architecture that contributes to both continuous and discrete phenotypic variation supports a more gradual selection model for the evolution of paedomorphosis.
Phenotypic evolution, diversification and historical patterns of community assembly are typically analysed with molecular phylogenies of extant species or the fossil record, but these two data types are rarely merged in single studies. While the utility of fossil taxa (when available) may be limited by incomplete information on diversity and ecology, they can provide refined estimates of ancestral trait reconstructions, and direct observations of coarse lineage composition and trait variation of ancestral communities [–]. In this study, we present a multi-locus nuclear phylogeny and divergence time estimates for all recognized extant species of amphiumids, proteids and sirenids. We placed the North American fossil taxa from these three families and two extinct families (Batrachosauroididae and Scapherpetontidae) on a time calibrated phylogeny, and used it to first reconstruct the origin and diversity of the SECP aquatic salamanders. We then used vertebral and body sizes of modern species to estimate the sizes of extinct species. Body size evolution (as a proxy for niche evolution) was then reconstructed to test the degree of conservation, liability or convergence of these traits between ancestral and modern species, and also to test if size diversification of modern species occurred before or after colonization of the SECP. Finally, we analysed the size structure, lineage composition and patterns of community reassembly during the evolution of this dynamic habitat through time.
What Salamanders Have Taught Us About Evolution | …
What Salamanders Have Taught Us About Evolution
with a robust phylogenetic hypothesis, is rampant in salamander ..
Salamanders and Newts
THE AMAZING WORLD OF SALAMANDERS - Scientific …
01/10/2013 · THE AMAZING WORLD OF SALAMANDERS
some pretty radical reversals have occurred in siren evolution
All plethodontid salamanders are lungless and breathe through moist skin. Plethodontids are unique among salamanders in having narrow grooves between each nostril and the upper lip. Males often have protuberances on the upper lip associated with the nasolabial grooves and a mental gland located beneath the mouth. Costal grooves are pronounced. Most plethodontids are completely terrestrial and lay eggs on land (subfamilies Bolitoglossinae and Plethdontinae and three species of the supergenus Desmognathus). Some have a biphasic life cycle with an aquatic larva and terrestrial adult (most members of the supergenus Desmognathus and subfamily Hemidactyliinae), and others are completely aquatic and permanently larval in form (some members of the subfamily Spelerpinae). Aquatic forms usually inhabit streams.
Lungless Salamander Family, the Plethodontidae
FIGURE 4. Three different salamander life histories: (A) aquatic adult salamander with larva's gills and tail fin; (B) aquatic larva with gills and tail fin that becomes a terrestrial adult with no gills or tail fin; (C) fully terrestrial salamander with larva that loses its gills and fin when it hatches from the egg. Illustration drawn by Carole B.
Some have a biphasic life cycle with an aquatic ..
Gao and Shubin (2001) investigated higher-level salamander relationships using combined morphological and molecular (nuclear ribosomal RNA from Larson and Dimmick, 1993) data, placing three of four families comprised of paedomorphic species in a single clade. In contrast, previous phylogenies (e.g., Duellman and Trueb, 1986; Larson and Dimmick, 1993) indicate that salamanders that retain at least some aquatic larval characteristics throughout life (families Amphiumidae, Cryptobranchidae, Proteidae, Sirenidae and parts of the Ambystomatidae and Plethodontidae) do not form a monophyletic group, and thus evolutionary displacement of larval characteristics into the adult phase of the life cycle must have occurred independently multiple times in salamanders.
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