|
''Oryzomys'' is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat (''O. palustris'') of the United States and ''O. couesi'' of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name ''Oryzomys'' was established in 1857 by Spencer Fullerton Baird for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in ''Oryzomys''. Species of ''Oryzomys'' are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to buff. The animals have broad feet with reduced or absent ungual tufts of hair around the claws and, in at least some species, with webbing between the toes. The rostrum (front part of the skull) is broad and the braincase is high. Both the marsh rice rat and ''O. couesi'' have 56 chromosomes, lack a gall bladder, and have a complex penis (as is characteristic of the Sigmodontinae) with some traits that are rare among oryzomyines; these characteristics are unknown in the other species of this genus. The habitat includes various kinds of wetlands, such as lakes, marshes, and rivers. ''Oryzomys'' species swim well, are active during the night, and eat both plant and animal food. They build woven nests of vegetation. After a gestation period of 21 to 28 days, about four young are born. Species of ''Oryzomys'' are infected by numerous parasites and carry at least three hantaviruses, one of which (Bayou virus) also infects humans. Two, maybe three, species have gone extinct over the last two centuries and at least one other is endangered, but the widespread marsh rice rat and ''O. couesi'' are not threatened. ==Taxonomy== ''Oryzomys'' is one of about thirty genera within the tribe Oryzomyini, a diverse group of well over a hundred species, many of which were formerly also included in ''Oryzomys''.〔Weksler, 2006, pp. 1, 10; Weksler et al., 2006, p. 1, table 1〕 Oryzomyini is one of several tribes within the subfamily Sigmodontinae of the family Cricetidae, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia.〔Musser and Carleton, 2005〕 Within Oryzomyini, a 2006 phylogenetic analysis by Marcelo Weksler which used both morphological and DNA sequence data found some evidence that ''Oryzomys'' is most closely related to a group including ''Holochilus'', ''Lundomys'', and ''Pseudoryzomys''. Although analyses based on morphological and combined data supported this relationship, sequences of the ''Rbp3'' gene alone instead placed ''Oryzomys'' among a group that included ''Nectomys'', ''Sigmodontomys'', and a few other genera. In all analyses, ''Oryzomys'' appeared within clade D of Oryzomyini.〔Weksler, 2006, figs. 34–39〕 The relationship between ''Oryzomys'' and the ''Holochilus'' group was supported by five synapomorphies (shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tufts of hairs surrounding the claws; having the back margin of the zygomatic plate of the skull at the same level as the front of the first upper molar; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that ''Oryzomys'' and the members of the ''Holochilus'' group share, and may thus be examples of convergent evolution.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Oryzomys」の詳細全文を読む スポンサード リンク
|