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Old 10-27-2017, 01:51 PM
Jeremy V Jeremy V is offline
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Some light reading from Miller (1972)

Origin. In speculating on the origin of Salmo apache, it is necessary to discuss also the probable or likely derivation of the other western North American trouts that have a similarly restricted distribution (S. aguabonita, S. gilae, and S. chrysogaster). The derivation of the three southernmost species is treated separately from that of S. aguabonita. Pertinent to this discussion is the occurrence of a fossil trout (Salmo sp.) from the vicinity of Lago de Chapala, Mexico (20? 15' N. Lat., 103? 00' W. Long.), about 400 airline km farther south than any living native trout has been reported. The present climate of the region, which is 1,500 m above sea level, is sub-humid and sub-tropical. The remains (being studied by Ted M. Cavender), kindly loaned by Dr. Jos6 Alvarez (Escuela Nacional de Ciencias Biologicas, I.P.N., Mexico City), came from the north side of the lake at Ajijic, but the precise locality is unknown. Possibly they are from the Chapala formation (Downs, 1958), which is either of early Pleistocene orlate Pliocene age or both (Clements, 1963). The remains (of a fish over 0.6 m long) include an incomplete skull and several vertebrae, of which the latter have the smoothsided walls found in Salmo chrysogaster but in no other living species of North American Salmo (see Osteological Notes, above). Four other Salmo vertebrae from Los Angeles County Museum of Natural History Loc. 65191 (Lago de San Marcos, just west of Lago de Chapala on east side of El Tecolote), being studied by W. I. Follett, represent a species different than that from Chapala. The San Marcos vertebrae more closely resemble those of Salmo clarki (Follett, pers. comm., 1968; photographs of the vertebrae examined by Ted Cavender and myself who concur), possessing much the same ornamentation of the lateral walls as noted in the cutthroat trout. The age of the beds from which the San Marcos Salmo was recovered is Late Pleistocene (Howard, 1969:7). Obviously more and better material of fossil Salmo from this area is needed before the full significance of these two finds can be determined. It looks, however, as if the Salmo chrysogaster evolutionary line may have been established at least three million years ago (during Plio-Pleistocene time-see Flint, 1971:3), probably much earlier, and that a trout resembling S. clarki also occurred far south in Mexico in Pleistocene time. In this connection it is of interest to note that Cope (1886) reported young trout from southern Chihuahua, Mexico, with ". . . teeth on the basihyal bones, as Salmo purpuratus, which they otherwise resemble." This implies that these fish were S. clarki (unknown today in Mexico) but the specimens have not been found and their identity must remain uncertain.

The origins of Salmo apache and S. gilae must be considered in the light of current knowledge of the late Cenozoic hydrography and geological history of Arizona and adjoining areas, particularly the regions now occupied by the lower Colorado River and the Gila River (Fig. 1).

The Colorado River was not a throughflowing stream, through what is now Grand Canyon to the Gulf of California, until the latter part of the Pliocene (Hunt, 1969:67, 113-119), perhaps 4 million years ago or earlier. At that time the Gulf of California extended far north of the present river mouth (Metzger, 1968). In order to account for the possession of certain traits shared by Arizona, Gila and Mexican golden trouts (few vertebrae and pyloric caeca, golden yellow color, yellow to orange "cutthroat" mark), it may be hypothesized that during the Pliocene the primitive Salmo chrysogaster or its antecedent was more widely distributed, occurring at least as far north as the region of the lower Colorado River basin, and that this northern stock there met a primitive cutthroat trout with which it hybridized to produce an ancestral form from which S. gilae and perhaps S. apache were derived. The latter might have differentiated from this ancestral stock above barrier falls in the Salt River. Needham and Gard (1964), Behnke (1970), and Schreck and Behnke (1971) hold that S. aguabonita is part of an evolutionary line including the above three trouts, but I present an alternate hypothesis below to account for the origin of aguabonita.

A more plausible origin for Salmo apache, however, is as follows. Sometime during the Pliocene an ancestral cutthroat trout or representative of that phyletic line moved into what is now the Little Colorado River basin from the region of the upper Colorado (Fig. 1). At that time the antecedent of the Little Colorado was an interior stream since Grand Canyon did not exist (McKee et al., 1967). The ancestral stock became isolated in the Little Colorado basin and evolved into S. apache, later gaining access to upper tributaries of Salt River (and San Francisco River, as explained earlier) by natural stream connections in the White Mountains around Mount. Ord. Historically, populations of typical Salmo clarki from the Colorado basin (Fig. 1) are not known to have occurred lower down in that drainage than the headwaters of the Dirty Devil River (a skin, USNM 110378, from Seven Mile Creek, Sevier Co., Utah, was taken on 27 June 1923, by S. B. Locke, who stated: This is ". . . the last stream of any importance entering the Colorado River from the west which contains trout." The specimen has about 205 scales in the lateral series). However, since cutthroat trout were native to the San Juan River, Colorado (Fig. 1), the species probably once occurred as far down the Colorado as the mouth of the San Juan. I have previously discussed the "records" of S. clarki pleuriticus from Salton Sea, California, concluding that they probably represented rainbow trout (Miller, 1950:27). I favor the origin of S. apache in the Little Colorado drainage because this hypothesis has the added support of the chromosome data, which strongly suggest that the karyotype of S. apache is most readily derived from the cutthroat trout line. This remains to be tested when the karyotypes of S. gilae and S. chrysogaster become known.

A third possible explanation is that the basal stock that led to Salmo chrysogaster was segregated in the southern part of the range of western Salmo (Mexico to Arizona) and became isolated into two stocks, one in the region of the Little Colorado River and the other in the Gila River basin. One stock then evolved into S. apache, the other into S. gilae.

It has also been suggested (Needham and Gard, 1964:173) that Salmo gilae might have originated during the Pleistocene by hybridization between a primitive cutthroat and rainbow trout in the lower Colorado River basin. This could account for the mixture of cutthroat and rainbow traits found in this species, but this hypothesis seems to me to be a less likely explanation now than when it was proposed. Again, the karyotype of S. gilae may prove critical to an explanation for its origin.

Detailed studies of the comparative osteology, immunogenetics (and other biochemical approaches), and karyotypes of S. gilae, S. chrysogaster, and other western Salmo may lead to a clarification of the origin and relationships of these trouts. Pertinent to the problem is the status of the "redband" trout (first brought to my attention by Robert Behnke) that now occurs in certain isolated waters of northern California (Fig. 1) and Oregon (see discussion below).

The most parsimonious explanation for the origin of the golden trout of California (Salmo aguabonita) is that it was derived from an ancestral stock, or from a combination of ancestral forms, that entered the Sacramento-San Joaquin Valley from the north. The "redband" trout just mentioned, which resembles both S. clarki and S. aguabonita, may represent the vestige or derivative of such an ancestor. I have examined 25 specimens of this trout from Sheepheaven Creek in the drainage of the upper McCloud River, California (UMMZ 188817, 188847, 190815, the first ones kindly made available by R. J. Behnke and D. W. Seegrist), one from a tributary of Clover Creek in the North Fork of the Feather River (UMMZ 140228), and 17 fingerling to adult specimens from Rush Creek, Modoc County, California (UMMZ 130642, but these show evidence of hybridization with rainbow trout). This "redband" trout, seemingly represented by remnant (isolated) populations northward through southern Oregon, certainly looks more like a cutthroat than a rainbow; however, its position in the evolutionary history of western trout is not yet clear. Its chromosome number (2n = 58) is the same as that of S. aguabonita and the two appear to have identical karyotypes (compare Figs. 6 and 8, and see Table 2).

Many of the traits common to S. aguabonita and S. apache (and to the other "golden trouts") may simply represent primitive characters. Some of these characters (not all possessed by the trout just mentioned) are: (1) few vertebrae, (2) 9 pelvic rays, (3) basibranchial teeth, (4) 3 epurals, (5) weak or no ornamentation on the vertebral centra, and (probably) (6) yellow or golden life colors. Primitive features are not valid indicators for inferring the relationships of phyletic lines (Hennig, 1966:95).

Conclusions. There are at least four and possibly five phyletic lines of the genus Salmo in Western North America: (1) the Mexican trout, Salmo chrysogaster, which seems to represent the most primitive evolutionary group, (2) the cutthroat trouts, Salmo clarki, less specialized and generally more primitive than (3) the rainbow trouts, Salmo gairdneri, a more recently derived group, and (4) the "redband" trout, as yet undescribed, showing resemblances to Salmo clarki and the golden trout, Salmo aguabonita, and probably more primitive as well as older than S. aguabonita but not than S. clarki. S. apache probably was derived from the cutthroat line; the origin of S. gilae is uncertain, but perhaps it represents a fifth phyletic line. S. aguabonita appears to have arisen from the "redband" trout line. The karyotypes may be critical in determining the positions of S. chrysogaster and, especially, S. gilae. Salmo apache is accorded full specific status because it possesses a combination of characters (see Diagnosis) which readily distinguish it from all other western trouts.
(there may be some copy/paste typos in there....)

Also, Behnke's book "Native Trout of Western North America" certainly has some additional information...
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