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ScienceWeek
2004 4 June B1
1. DEVELOPMENTAL BIOLOGY: ON VERTEBRATE LIMB STRUCTURES
The following points are made by M.D. Shapiro et al (Nature 2004 428:717):
1) Vertebrate limb structures exhibit extensive structural variation in animals adapted to different environments. In a phylogenetically diverse set of vertebrates -- including lineages of reptiles, amphibians, marine mammals and fish -- hindlimb structures are characteristically reduced or missing. The presence of rudimentary hindlimb structures in snakes and whales formed part of the early evidence indicating that these animals evolved from four-limbed ancestors through extensive modification of pre-existing skeletal structures(1,2). Despite longstanding interest in the mechanisms that control hindlimb reduction during vertebrate evolution, the detailed number, location and type of genetic changes that underlie this process are still unknown(3-5).
2) Marine threespine sticklebacks (Gasterosteus aculeatus) have a prominent pelvic skeleton made up of bilateral pelvic spines that articulate in trochlear joints with an underlying pelvic girdle. The girdle covers part of the ventral surface and extends up the lateral side of the fish in an ascending branch that articulates with dermal armour plates. Previous studies suggest that pelvic structures protect sticklebacks against gape-limited, soft-mouthed predators by presenting a lacerating defensive structure, which increases the effective diameter of the fish and resists compressive forces during predator manipulation and chewing. However, several freshwater stickleback populations have evolved complete or partial loss of the pelvic skeleton, perhaps in response to local absence of predatory fish, reduced levels of calcium ion availability, or predation by macroinvertebrates that typically capture sticklebacks by grasping the dorsal and pelvic spines. The young geological age of the postglacial lakes containing pelvic-reduced sticklebacks, and the rapid tempo of pelvic changes seen in a high-resolution series of fossil sticklebacks, suggest that pelvic reduction can evolve in less than 10,000 generations in this system.
3) In summary: Hindlimb loss has evolved repeatedly in many different animals by means of molecular mechanisms that are still unknown. To determine the number and type of genetic changes underlying pelvic reduction in natural populations, the authors carried out genetic crosses between threespine stickleback fish with complete or missing pelvic structures. Genome-wide linkage mapping shows that pelvic reduction is controlled by one major and four minor chromosome regions. Pitx1 maps to the major chromosome region controlling most of the variation in pelvic size. Pelvic-reduced fish show the same left-right asymmetry seen in Pitx1 knockout mice, but do not show changes in Pitx1 protein sequence. Instead, pelvic-reduced sticklebacks show site-specific regulatory changes in Pitx1 expression, with reduced or absent expression in pelvic and caudal fin precursors. The authors suggest that regulatory mutations in major developmental control genes may provide a mechanism for generating rapid skeletal changes in natural populations, while preserving the essential roles of these genes in other processes.
References (abridged):
1. Darwin, C. The Origin of Species 450-456 (John Murray, London, 1859)
2. Struthers, J. On the bones, articulations, and muscles of the rudimentary hind-limb of the Greenland right-whale (Balaena mysticetus). J. Anat. Phys. 15, 141-176, 302-321 (1881)
3. Bejder, L. & Hall, B. K. Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss. Evol. Dev. 4, 445-458 (2002)
4. Lande, R. Evolutionary mechanisms of limb loss in tetrapods. Evolution 32, 73-92 (1978)
5. Cohn, M. J. & Tickle, C. Developmental basis of limblessness and axial patterning in snakes. Nature 399, 474-479 (1999)
Nature http://www.nature.com/nature
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ON THE ANCESTRY OF WHALES
The following points are made by Kenneth D. Rose (Science 2001 293:2216):
1) Whales are mammals that apparently moved to the sea approximately 50 million years ago, and exactly how whales are related to other mammals has long been one of the most vexing questions facing mammologists and paleontologists. In the last decade, mounting evidence that whales are highly specialized ungulates (hoofed mammals) has been bolstered by the discovery of an impressive array of previously unknown fossil whales in Pakistan, India, and Egypt, fossils that largely fill the morphological gulf between land mammals and ocean-dwelling cetaceans (whales, dolphins, and porpoises).
2) The move to the ocean required many adaptations to living in water, but the earliest whales still closely resembled land animals. One of the most spectacular transitional forms is the "walking whale" Ambulocetus from the middle Eocene (approximately 47 to 48 million years ago. This species had relatively well-developed limbs, paraxonic feet (where the plane of symmetry passes between the third and fourth digits), and hoof-like terminal toe bones.
3) But fossils have failed to provide conclusive indications of the closest relatives of whales, and instead have sparked new controversy. Most recent morphological analyses suggest that the mesonychians, an extinct group of terrestrial carnivorous ungulates, form the sister group of cetaceans. Molecular systematists, however, maintain that cetaceans belong to the artiodactyls (even-toed ungulates such as sheep, cows, pigs, camels, deer, and hippos) and are in fact the sister group of hippopotami.
4) Gingerich et al (Science 2001 293:2239) report important new fossil evidence -- skeletons of two very primitive ancient whales with well-developed limbs from the middle Eocene of Pakistan --that moves the dispute closer to resolution. The new fossils provide compelling morphological evidence that whales are not merely related to but descended from artiodactyls rather than from mesonychians, thus bringing the morphological evidence into agreement with molecular data, at least at the level of taxonomic orders.
Science http://www.sciencemag.org
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THE EARLIEST KNOWN FULLY QUADRUPEDAL SIRENIAN
The following points are made by D.P. Domning (Nature 2001 413:625):
1) Modern seacows (manatees and dugongs; Mammalia, Sirenia) are completely aquatic, with flipper-like forelimbs and no hindlimbs. Since 1990, abundant remains of sirenians, together with other marine taxa of early middle Eocene age (approximately 50 million years ago), have been collected from Seven Rivers, Jamaica. The sediments (silt-stones and sandstones) represent a lagoonal, estuarine, or deltaic depositional environment.
2) The author reports Eocene fossils from Jamaica that represent nearly the entire skeleton of a new genus and species of sirenian -- the most primitive for which extensive postcranial remains are known. This animal was fully capable of locomotion on land, with 4 well-developed legs, a multivertebral sacrum, and a strong sacroiliac articulation that could support the weight of the body out of water as in land mammals. Aquatic adaptations demonstrate, however, that this animal probably spent most of its time in the water. Its intermediate form thus illustrates the evolutionary transition between terrestrial and aquatic life. Similar to contemporary primitive cetaceans, the animal probably swam by spinal extension with simultaneous pelvic paddling, unlike later sirenians and cetaceans, which lost the hindlimbs and enlarged the tail to serve as the main propulsive organ. Together with fossils of later sirenians elsewhere in the world, these new specimens document one of the most marked examples of morphological evolution in the vertebrate fossil record.
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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