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PubMed2006

Interactions of gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH) in birds and mammals.

Gonadotropin-releasing hormone (GnRH) regulates secretion of both of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone. Thus, it is a key hormone for vertebrate reproduction. GnRH was considered to be unusual among hypothalamic neuropeptides in that it appeared to have no direct antagonist, although some neurochemicals and peripheral hormones (opiates, GABA, gonadal steroids, inhibin) can modulate gonadotropin release to a degree. Five years ago, a vertebrate hypothalamic neuropeptide that inhibited pituitary gonadotropin release in a dose-dependent manner was discovered in quail by Tsutsui et al. (2000. Biochem Biophys Res Commun 275:661-667). We now know that this inhibitory peptide, named gonadotropin-inhibitory hormone, or GnIH, is a regulator of gonadotropin release in vitro and in vivo. Its discovery has opened the door to an entirely new line of research within the realm of reproductive biology. In our collaborative studies, we have begun to elucidate the manner in which GnIH interacts with GnRH to time release of gonadotropins and thus time reproductive activity in birds and mammals. This paper reviews the distribution of GnIH in songbirds relative to GnRHs, and our findings on its modes of action in vitro and in vivo, based on laboratory and field studies. These data are simultaneously compared with our findings in mammals, highlighting how the use of different model species within different vertebrate classes can be a useful approach to identify the conserved actions of this novel neuropeptide, along with its potential importance to vertebrate reproduction.

Animals↗
PubMed2005

Evolution of the vertebral formulae in mammals: a perspective on developmental constraints.

Developmental constraints refer to biases that limit phenotypic changes during evolution. To examine the contribution of developmental constraints in the evolution of vertebrate morphology, we analyzed the distribution pattern of mammalian vertebral formulae. Data on mammalian vertebral formulae were collected from the Descriptive Catalogue of the Osteological Series Contained in the Museum of the Royal College of Surgeons of England by Richard Owen (1853) and were plotted onto the most reliable mammalian phylogenetic tree based on recent molecular studies. In addition to the number of cervical vertebrae that is almost fixed to 7, we found that the number of thoracolumbar vertebrae tends to be 19 in many groups of mammals. Since fidelity of the number of thoracolumbar vertebrae was also completely maintained in Monotremata and Marsupialia, we presumed that thoracolumbar vertebral number as well as cervical vertebral number might have been fixed in the primitive mammalian lineage. On the basis of primitive vertebral formulae, we could clarify the polarity of evolution and identify several deviations from the primitive states during the mammalian evolution. The changes in the vertebral formulae in eutherian mammals seem to be lineage-specific, such that most species in Carnivora have 20 instead of 19 thoracolumbar vertebrae. Because such lineage-specific vertebral formulae contrast with the estimated distribution pattern on the assumption of evolution only through the selective pressure, we concluded that developmental constraints played an important role in the evolution of mammalian vertebral formulae.

Animals↗
PubMed2005

Molecular evolution of evolutionary novelties: the vagina and uterus of therian mammals.

Innovations are an integral part of the evolutionary process if we accept the fact that more complex organisms derived from anatomically simple ones. All major taxa are distinguished not only by their closer genealogical relatedness relative to other species but also by the possession of novel anatomical and physiological features. The question is whether the origin of these novel characters can be simply understood as adaptations, like all other phenotypic differences that arise by natural selection, or whether the origin of these characters requires more profound genetic changes. In this paper, we argue that innovations constitute a distinct class of evolutionary processes that require a research program complementary to the study of adaptation. The distinguishing feature of innovations is the origin of novel organ identity gene functions specific to the novel character. By implication, research into the origin of novel characters has to identify the developmental regulatory links that were involved in the evolution of these characters. We suggest that novel regulatory links will include the evolution of cis-regulatory elements as well as novel protein-protein interactions among transcription factor proteins. The latter hypothesis suggests that innovations should leave a trace in the evolution of the protein coding regions of transcription factor genes. We illustrate this idea with results on the evolution of HoxA-11 and HoxA-13 in the stem lineage of placental mammals. These genes are essential for female reproductive tract development and function. We show that, as predicted, these genes experience strong directional selection in the stem lineage of placental mammals and that these amino acid substitutions affect residues at the surface of the protein, consistent with their expected role in protein-protein interactions. We conclude that a careful analysis of sequence variation in developmental genes can aid in testing which developmental changes were instrumental in the origin of novel morphological characters.

Animals↗
PubMed2003

Viviparous lizard, Eulamprus tympanum, shows changes in the uterine surface epithelium during early pregnancy that are similar to the plasma membrane transformation of mammals.

The "plasma membrane transformation" describes a series of ultrastructural, biochemical, and morphological changes that occur in the uterus of many mammals at the time of blastocyst attachment. These changes, regardless of placental type or length of gestation, include alterations to microvillar length and density and the presence or absence of pinopods or uterodomes. Scanning electron microscopy (SEM) was used to 1) document the topographical ultrastructure of the uterus of Eulamprus tympanum, an eastern Australian viviparous skink with a simple chorioallantoic placenta, for the first time; and 2) determine whether changes identified as "plasma membrane transformation" in mammals occur in E. tympanum. Tissues collected over three seasons from nonreproductive subadult females, preovulatory, postovulatory, and early to mid-gestational females were examined. At low magnification the uterine epithelium of subadults displays a distinctive pattern of tissue folding that includes rectangular areas of tissue delineated by deep lateral and transverse folds. At higher magnification, the uterine epithelium surface is composed of two dominant cell types, i.e., those covered by microvilli and ciliated cells. The folding pattern observed in subadults is less evident in vitellogenic females and the cell surfaces appear highly secretory, with bulging cell apices. Tissue from postovulatory lizards has no distinctive folding pattern and cell surfaces are frequently smooth and lack microvilli. Uterine egg chambers lack ciliated cells at the embryonic pole, but display abundant secretory droplets. Thus, the uterus of E. tympanum undergoes a plasma membrane transformation. The scope of this transformation is not fully understood but may be related to the complexity of placental structure and the development of the embryo/fetus at parturition.

Animals↗
PubMed1980

Morphology of the gastrointestinal tract in primates: comparisons with other mammals in relation to diet.

Three categories of dietary adaptation are recognized--faunivory, frugivory, and folivory--according to the distinctive structural and biochemical features of animal matter, fruit, and leaves respectively, and the predominance of only one in the diets of most species. Mammals subsisting mainly on animal matter have a simple stomach and colon and a long small intestine, whereas folivorous species have a complex stomach and/or an enlarged caecum and colon; mammals eating mostly fruit have an intermediate morphology, according to the nature of the fruit and their tendency to supplement this diet with either animal matter or leaves. The frugivorous group are mostly primates: 50 of the 78 mammalian species, and 117 of the 180 individuals included in this analysis are primates. Coefficients of gut differentiation, the ratio of stomach and large intestine to small intestine (by area, weight, and volume), are low in faunivores and high in folivores; the continuous spread of coefficients reflects the different degrees of adaptation to these two dietary extremes. Interspecific comparisons are developed by allowing for allometric factors. In faunivores, in which fermentation is minimal, the volume of stomach and large intestine is related to actual body size, whereas these chambers are more voluminous in larger frugivores and mid-gut fermenting folivores; fore-gut fermenters show a marked decrease in capacity with increasing body size. Surface areas for absorption are related to metabolic body size, directly so in frugivores; area for absorption is relatively less in larger faunivores and more in large folivores, especially those with large stomachs. Indices of gut specialization are derived from these regressions by nonlinear transformation, with references to the main functional features of capacity for fermentation and surface area for absorption. These are directly comparable with the dietary index, derived from quantitative feeding data displayed on a three-dimensional graph, with all species within a crescentic path from 100% faunivory through 55--80% frugivory to 100% folivory, perhaps illustrating, at least for primates, the evolutionary path from primitive insectivorous forms through three major ecological grades.

Animals↗
PubMedDate not supplied

Exponential analysis of the lung pressure-volume curve in newborn mammals.

The compliance of the lung (per unit of lung weight) is less in newborn mammals than in adults. This could result from a smaller volume of airspaces per unit weight and/or a lower lung distensibility. The isolated role of lung distensibility was evaluated by using a mathematical description of the pressure-volume (P-V) curve during lung deflation. Deflation limbs of static P-V curves in newborns of six species (four experimentally obtained and two taken from the literature) ranging from total lung capacity to the resting volume (Vr) were fitted by a monoexponential function of the type V = B - Ae-KP, where B equals Vmax at infinite P, A equals the difference between Vmax and V at P = O, and K is a constant representing lung distensibility. Unlike in adults, in newborns the monoexponential fitting provided an adequate description of the P-V curve for only a relatively small range of transpulmonary pressure (from P at Vr to 10-15 cm H2O). The K value of this portion of the curve was similar among species but higher than in adult mammals, averaging 0.240 cm H2O-1. This suggests a similar lung structure in the different species. Since lung distensibility in newborns is larger than in adults, the fact that a unit mass of lung in the newborn is less compliant should be due to the smaller volume of its airspaces.

Age Factors↗
PubMed1993

Molecular cloning of a novel, putative G protein-coupled receptor from sea anemones structurally related to members of the FSH, TSH, LH/CG receptor family from mammals.

Using oligonucleotide probes derived from consensus sequences of known vertebrate and invertebrate G protein-coupled receptors, we have cloned the cDNA for a presumed G protein-coupled receptor from sea anemones. This receptor shows a striking structural homology with members of the glycoprotein hormone (FSH, TSH, LH/CG) receptor family from mammals, including a very large, extracellular N terminus (18-25% sequence identity) and a 7 transmembrane region (44-48% sequence identity). As with the mammalian glycoprotein hormone receptor genes, the sea anemone receptor gene yields transcripts which can be alternatively spliced, thereby yielding a shortened receptor variant only containing the large extracellular (soluble) N terminus. All this is strong evidence that the putative glycoprotein hormone receptor from sea anemones is evolutionarily related to those from mammals. This is the first report showing that a putative glycoprotein hormone receptor occurs in invertebrates.

Amino Acid Sequence↗
PubMed1997

The candidate sex-reversing DAX1 gene is autosomal in marsupials: implications for the evolution of sex determination in mammals.

The human X-linked DAX1 gene was cloned from the region of the short arm of the human X found in duplicate in sex-reversed Xdup Y females (E. Zanaria et al., 1994, Nature 372: 635-641). DAX1 is suggested to be required for ovarian differentiation and to play an important role in mammalian sex determination or differentiation pathways. Its proposed dose-dependent effect on sexual development suggests that DAX1 could represent an evolutionary link with an ancestral sex-determining mechanism that depended on the dosage of an X-linked gene. Furthermore, DAX1 could also represent the putative X-linked switch gene, which independently controls sexual dimorphisms in marsupial mammals in an X-dose-dependent manner (D.W. Cooper et al., 1993, Semin. Dev. 4: 117-128). If DAX1 has a present role in marsupial sexual differentiation or had an ancestral role in mammalian sex determination, it would be expected to lie on the marsupial X chromosome, despite the autosomal localization of other human Xp genes. We therefore cloned and mapped the DAX1 gene in the tammar wallaby (Macropus eugenii). DAX1 was located on wallaby chromosome 5p near other human Xp genes, indicating that it was originally autosomal and that it is not involved in X-linked dose-dependent sex determination in an ancestral mammal nor in marsupial sexual differentiation.

Amino Acid Sequence↗
PubMed2000

Middle and Later Stone Age large mammal and tortoise remains from Die Kelders Cave 1, Western Cape Province, South Africa.

Die Kelders Cave 1, South Africa, has provided more than 150,000 taxonomically identifiable mammal and tortoise bones from Middle Stone Age (MSA) and Later Stone Age (LSA) deposits. Cape dune mole-rats dominate the mammal sample, and they appear to have been accumulated mainly by people during the LSA occupation and mainly by eagle owls in the MSA. In sharp contrast to the LSA fauna, the MSA sample contains extralimital ungulates that imply relatively moist, grassy conditions. The large mean size of the MSA mole-rats also points to greater humidity, while the large size of the gray mongooses implies cooler temperatures. The sum supports luminescence and ESR dates that place the MSA occupation within the early part of the Last Glaciation (global isotope stage 4). The Die Kelders ungulate bones support those from Klasies River Mouth in suggesting that MSA people obtained dangerous terrestrial prey much less frequently than their LSA successors, probably because MSA people lacked the bow and arrow and other projectile weapons. The Die Kelders tortoise bones constrain the extent of climatic change, since their abundance indicates that warm, dry days remained common, at least seasonally. The tortoises tend to be much larger in the MSA layers than in the LSA ones, suggesting that MSA people collected tortoises less intensively, probably because MSA populations were relatively sparse.

Animals↗
PubMed2001

Integrative study on chromosome evolution of mammals, ants and wasps based on the minimum interaction theory.

There is well-known evidence that in many eukaryotes, different species have different karyotypes (e.g. n=1-47 in ants and n=3-51 in mammals). Alternative (fusion and fission) hypotheses have been proposed to interpret this chromosomal diversity. Although the former has long been accepted, accumulating molecular genetics evidence seems to support the latter. We investigated this problem from a stochastic viewpoint using the Monte Carlo simulation method under the minimum interaction theory. We found that the results of simulations consistently interpreted the chromosomal diversity observed in mammals, ants and wasps, and concluded that chromosome evolution tends to evolve as a whole toward increasing chromosome numbers by centric fission. Accordingly, our results support the fission hypothesis. We discussed the process of chromosome evolution based on the latest theory of the molecular structure of chromosomes, and reconfirmed that the fission burst is the prime motive force in long-term chromosome evolution, and is effective in minimizing the genetic risks due to deleterious reciprocal translocations and in increasing the potential of genetic divergence. Centric fusion plays a biological role in eliminating heterochromatin (C-bands), but is only a local reverse flow in contrast to the previously held views.

Animals↗
PubMed1996

Molecular evolution and phylogenetic utility of the polyubiquitin locus in mammals and higher vertebrates.

The product of ubiquitin genes is a small protein involved in intracellular sorting of other proteins. The locus consists of tandemly arrayed, uninterrupted copies of the gene. As several studies have noted, the Polyubiquitin locus is a model system for studying concerted evolution. While the protein is among the most conserved known, individual copies within an organism show variation in nucleotide sequence despite clear evidence of concerted evolution. When treated as individuals, repeats from a given locus form a monophyletic group. Furthermore adjacent copies often cluster, suggestive of the mechanism of concerted evolution. Due to this concerted evolution of repeats (and loci in organisms with multiple polyubiquitins), sequencing of heterogeneous PCR products consisting of all the repeats in a given organism may yield phylogenetic signal, as with other multicopy genes. We test this possibility through 22 original sequences using primers designed so that only tandem copies are amplified. Using these and previously published data, we further explore these phenomena in higher vertebrates and mammals in particular. We suggest that multiple locus duplications have occurred within mammals. Positional codon bias is strongly evident. We also find substitutional bias with regard to codon type. GC content of the locus appears to be generally high across vertebrates. Intraorganismal variation is tallied as an indication of frequency of change in codon position and transition/transversion ratios to further elucidate the tempo and mode of molecular evolution. Using these data, a weighting scheme for ubiquitin is also presented. Despite the gene's high GC content, transitional changes still appear more frequent. While the phylogenetic utility of ubiquitin does not appear great, its mechanistic insights seem far from exhausted.

Amino Acid Sequence↗
PubMed1996

A congruence study of molecular and morphological data for eutherian mammals.

The four orders of eutherian mammals which are traditionally placed in the superorder Archonta [Chiroptera (microbats and megabats), Dermoptera (flying lemurs), Primates (primates), and Scandentia (tree shrews)] are among the best-studied taxa of their infraclass from both the molecular and morphological perspectives. Nevertheless, the ordinal relationships of archontans remain unresolved. While morphological studies favor their monophyly, molecular investigations do not. To evaluate these opposing conclusions, parsimony analyses were conducted with three separate sets of DNA sequences from both the nuclear and mitochondrial genomes and one file of morphological data for archontans and other eutherian mammals. Statistical tests of character support and ordinal branching pattern differences documented that the three sets of DNA sequences and their results were homogeneous and congruent, thereby allowing for the combination of these data into one large matrix for further phylogenetic analysis. In contrast, these same tests revealed that the combined sequence and morphological files and their topologies were in strong conflict. Archontan monophyly was supported by the morphological evidence, but this arrangement was strongly rejected by the combined DNA sequences that favored instead a grouping of Dermoptera, Primates, and Scandentia with Lagomorpha (rabbits) and Rodentia (rodents). Resolution of these significant differences will require further evaluations about the homologies and evolutionary properties of the molecular and morphological characters and about the appropriateness of the chosen phylogenetic methods, as well as the incorporation of new comparative data from both sources.

Animals↗
PubMed1998

Higher taxonomic relationships among extant mammals based on morphology, with selected comparisons of results from molecular data.

Until a few decades ago, phylogenetic relationships among placental orders were ambiguous and usually depicted to radiate as an unresolved "bush." Resolution of this bush by various workers has been progressing slowly, but with promising results corroborated by nondental, dental, and molecular characters. In this study we continue to seek resolution. A total of 258 nondental and 2 dental characters was analyzed by PAUP and MacClade on 39 vertebrate taxa (3 reptiles, 1 nonmammalian therapsid, and 35 mammals; 20 of the mammals are extant and 15 are extinct) to study higher taxonomic relationships with emphasis on Placentalia (Eutheria). About two-thirds of the characters are osteological, the rest concern soft tissues, including myological but excluding molecular characters (most are our data, the rest are from the literature). Cladistic analysis included all 39 taxa (fossil taxa help to evaluate polarities of characters) and all characters were given equal weight. Extant Mammalia are divided into Prototheria and Theria, the latter into Marsupialia and Placentalia. Placentalia comprises Xenarthra and Epitheria. Within Epitheria, Lipotyphla and Preptotheria (emended) are sister-taxa. Preptotherian taxa group into: ungulate-related taxa and various nonungulates. The former include Carnivora, Pholidota, Tubulidentata, Artiodactyla, Cetacea, Perissodactyla, Hyracoidea, Proboscidea, and Sirenia. A possible association to embrace Lagomorpha, Rodentia, Macroscelidea, Scandentia, Primates, Chiroptera, and Dermoptera is suggested. Significant differences between our findings and those of recent investigators include the dissociation of Pholidota from Xenarthra and the plesiomorphous position of Lipotyphla within Epitheria. Congruence between morphological and molecular results is closer than previously reported.

Animals↗
PubMed2004

Masculinization of female mammals: lessons from nature.

Although varying degrees of genital masculinization are a reasonably common phenomenon in the world of female mammals, the majority of such variation has not been investigated. In this chapter we have described research on the "masculinized" genitalia of moles and hyenas. Such research raises intriguing possibilities regarding the coordinated role that androgens, estrogens and peptide hormones (e.g., relaxin) might play, at different stages of sexual differentiation and development, in preparing genital tissues for their functional roles in reproduction. Such studies also suggest that non-androgenic mechanisms need to be considered. Arnold (1996) and Carruth et al. (2002) have recently presented the argument for broadening our view of sexual differentiation of brain and behavior, emphasizing direct genetic effects. A similar view has been presented for the Tammar wallaby, where formation of a scrotum, or a pouch, is a direct consequence of the presence/absence of two X chromosomes (Pask and Renfree, 2001). Although our research on moles and hyenas has not yet yielded such definitive results, the research reviewed in this chapter calls attention to processes that could well operate in other mammals, including humans.

Animals↗
PubMed1993

The 'individualization' of large North American mammals.

The enforcement of wildlife laws and the captive breeding of threatened/endangered species requires the ability to identify individual animals. DNA profiles of a variety of large North American mammals, birds, and fish were generated using ten different oligonucleotide probes. The probes tested were four multilocus probes [33.6, 33.15, JE46, and (TGTC)5] and six 'human unilocus' probes [MS1 (D1S7), CMM101 (D14S13), YNH24 (D2S44), EFD52 (D17S26), TBQ7 (D10S28), and MS43 (D12S11). Each of the probes was chemically synthesized, and labeled by the attachment of alkaline phosphatase; after hybridization, the probes were detected by chemiluminescence catalyzed by the enzyme. Initial screening against zoo blots including samples of bear, wolf, large cat, wild sheep, deer, birds, marine mammals, and fish indicated that three multilocus probes [33.15, 33.6, (TGTC)5] gave informative patterns containing 15-40 bands for most or all of the animals tested, as did two of the 'human unilocus' probes (MS1 and CMM101). The other five probes appeared informative only in some species (for example, YNH24 against canids). Subsequent screenings of populations within species were used to determine genetic diversity by analysis of observed bandsharing (S). Large heterologous populations, such as white-tailed deer, exhibited highly diverse band patterns (S < or = 0.2). Geographically isolated and/or genetically constricted animals, such as endangered Mexican wolves, Tule elk, and Columbian white-tailed deer, exhibited much higher frequencies of bandsharing (0.6 < or = S < or = 0.95).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1997

Intestinal absorptive surface in mammals of different sizes.

The present treatise is primarily concerned with the structural and morphometric parameters of the cecum and large intestine of mammals. Over the past century, numerous accounts have visually presented the variation and diversity of the large intestine of mammals. This includes above all comprehensive works on the macroscopic anatomy. More recently, the microscopic anatomy of various animals at the light, electron, and scanning electron microscopy levels has been covered, especially for rodents and primates. In the past two decades, progress has been made by adding a new dimension to the previous structural studies, namely, the morphometrical analysis of the intestine of various animals and subsequent employment of this data in an analysis based on principles of scaling. The present account follows essentially this outline, but presented in a slightly different order. First, in the introductory section, the scientific aims and general prologue to this field of study are presented. Included in this section is a short, concise literature survey that deals with the major literature available on the subject of the large intestine at the macroscopical and microscopical level, as well as the most recent morphometric analyses of the intestines. The main focus of the present work is on the methodological; a new method is described to measure the intestines of animals ranging in size from the harvest mouse to the horse. This technique may also be applicable to other hollow or tube-like organs. Heretofore, previous techniques have been based on obtaining the area of the surface mucosa by measuring lengths and widths and calculating the area by multiplying the two measurements. Alternatively, some methods have taken probes and made measurements at the light microscopy level and then extrapolated these results to determine the entire area. The former method is inadequate, while the latter possesses the inherent disadvantages of all sampling techniques. The present technique has two levels: (1) obtaining the basal surface area of the entire intestine and (2) accounting for any increase in the mucosal surface area that is due to microscopically visible folds, villi, or other such structures. The former is accomplished by flattening appropriately sized pieces of intestine between two glass plates and tracing the contours onto transparent paper. The entire intestine is processed in this manner, resulting in a basal surface area, the contours having been submitted to analysis on a semi-automatic image analyzer to determine the area in square millimeters. The second-level measurements determine a factor of surface enlargement by calculating a ratio of the distance along the surface contour of the mucosa on a histological section cut perpendicular to the width of the intestine to a second distance (reference line) drawn straight beneath the mucosa but not tracing the enlargements. The measurements obtained at both levels are multiplied to give a final surface area. In addition to a detailed description of the technique complete with a flow sheet and pictorial diagram, the various aspects of proper fixation, tissue embedding, shrinkage, and determination of sampling sites (for the second level of measurement) are discussed. A pilot experiment to determine the surface enlargement due to microvilli is presented from material taken from the giant pouched rat. This was performed by measuring video sequences of microvilli taken from electron microscopy images. Cecal microvilli increase the surface area 15-fold, while in the colon the increase is approximately 19- to 20-fold. In the discussion, the choice of using three animals per group is discussed, based on simple statistical tests. Section 3 is entitled "Morphology of the Mammalian Cecum." Chronologically, it marks the onset of the entire investigation. Before having developed the method described in Sect. 2, these morphological investigations at the light, electron and scanning electron microscopy

Animals↗
PubMed1993

Small and medium sized mammals as predators of ticks (Ixodoidea) in South Africa.

A total of 1,640 stomach contents from 56 South African species of small and medium sized mammals were examined for ticks. Only six ticks were found in three specimens of the caracal Felis caracal. Although identification of the tick species was not possible owing to fragmentation and partial digestion, the genus Haemaphysalis was represented in four of these cases. We conclude that predation by small and medium sized mammals on ticks is uncommon in South Africa.

Animals↗
PubMed1994

Evolutionary analysis of cytochrome b sequences in some Perciformes: evidence for a slower rate of evolution than in mammals.

To obtain information relative to the phylogenesis and microevolutionary rate of fish mitochondrial DNA, the nucleotide sequence of cytochrome b gene in seven fish species belonging to the order of Perciformes was determined. Sequence analysis showed that fish mitochondrial DNA has a nucleotide compositional bias similar to that of sharks but lower compared to mammals and birds. Quantitative evolutionary analysis, carried out by using a markovian stochastic model, clarifies some phylogenetic relationships within the Perciformes order, particularly in the Scombridae family, and between Perciformes, Gadiformes, Cypriniformes, and Acipenseriformes. The molecular clock of mitochondrial DNA was calibrated with the nucleotide substitution rate of cytochrome b gene in five shark species having divergence times inferred from paleontological estimates. The results of such analysis showed that Acipenseriformes diverged from Perciformes by about 200 MY, that the Perciformes common ancestor dates back to 150 MY, and that fish mitochondrial DNA has a nucleotide substitution rate three to five times lower than that of mammals.

Animals↗