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<br />210
<br />
<br />II ubbs and Miller
<br />
<br />Hybridization between Catostomus and Xyrauchen
<br />
<br />211
<br />
<br />. .
<br />
<br />moderately developed testes, and four females, probably spent, 379-
<br />H2 mm. long, all collected by W. F. Sigler and party in Green River
<br />at Hideout Canyon, nine miles from Vernal Road Junction, Daggett
<br />County, Utah (near the Wyoming border); August 5, 1950 (three
<br />specimens, 384, 386, and 442 mm. long, had labels reading "Spring
<br />Gr.," but are said by Sigler to have been taken in Hideout Canyon or
<br />p()s~ibly in Sheep Creek, a tributary of the Green River in the same
<br />region).
<br />
<br />Specimens in the two collections
<br />
<br />Catostomus
<br />latipinnis
<br />
<br />Hybrids
<br />
<br />X yrauchen
<br />
<br />j~eXanus
<br />
<br />general region and no doubt lives commonly in the. river where ~he
<br />hybrid was caught. In the series containing the SI~ large hyb.flds
<br />(V.M.M.Z., No. 162334, and U.M., No. 1~342), as It was receIved
<br />at the University of Michigan from prevIOusly scattered sources,
<br />there are forty-three specimens of C. latipinnis and none of X.
<br />texanus. The collection was made in a deep, swift part of the river,
<br />where representative series would be difficult to get, and the pre-
<br />served sample was perhaps selected. The proportional numbers of
<br />Xyrauchen (Table I) are probably not closely indicati~e of the act~al
<br />population, because of the humpback sucker's habI~of,ret~~mng
<br />to deep waters soon after spawning. Th~;rh~nW.?~S1,r;;i%~s~.very"..
<br />., abundant in the river channels" of the, upper,i.~?~~r~,~?,!f,~j,;~~~ystem); ,
<br />::,',',",' ,"1889',,:"(Jo,r,'dari' 1891 p. 26) but J6'cal"'"te.s"t",,I,m, 0, "n,y, "',1".1'"0,, m, John,T.
<br />, In ", " .,'" .~'.1Iti'"., """ , . . I
<br />~ Greenbank'an.ddothers, indicates that iP has"beco~~::imcreasmg y
<br />scarce ther~ intecentyears. ,W. P. Knoch ~astold us that am.ong
<br />''"about seven hundred suckers caught in 1946 m the C.olorado Rlv~r
<br />at the mouth of the Gunnison, only seven were Xyrauchen. ThIS
<br />great decrease in the numbers of the one species, in the prese~ce of an
<br />abundant population of thc other, has probably been It ~aJor cause
<br />of increased frequency of hybridization. Seven of the Clght kn?wn
<br />hybrids between C. latipinnis and X. texanus were coll~cted SInce
<br />X yrauchen has become scarce in the upper. waters. It I~ prob~ble
<br />that the ratio of hybrids to Xyrauchen IS markedly mc~easmg,
<br />though the ratio of hybrids to C. latipinnis presumably remams low,
<br />almost surely well below 1: 100 (except locally, perhaps).
<br />
<br />TABLE I
<br />
<br />Rl:LATIVE NUMBERS OF CATOSTOMUS LATIPINNIS, HYBRIDS, AND XYUAUCHEN
<br />TEXANUS
<br />The data are based on specimens in the United States National Mmleum and in
<br />the 1\luseum of Zoology of the University of Michigan. All specimens were examined
<br />by one or both authors. See text for discussion of ratios.
<br />
<br />I Per-
<br />No.
<br />centnge
<br />
<br />No.
<br />
<br />Per-
<br />centage
<br />
<br />N Per-
<br />o.
<br />centllg'~
<br />
<br />In coli eclion containing hybrid
<br />V.S N.M., No. 75992 . . . . . . . . . . . . 83 94.~ 1 ~.9 ] ~.9
<br />U.S. N.M., No. 148594 ........... 0 0 1 50.0 1 50.0
<br />U.l\. LM.Z., No. 16~334 et. al. .... . 45 88.2 6 11.8 (I 0
<br />From entire upper Colorado Rive]
<br />SJ stem (above Grand Canyon) . ' 816 97.5 8 1.0 I 18 1.6
<br /> :
<br />
<br />RELATIVE NUMBERS OF HYBRIDS AND PARENTAL SPECIljiS
<br />
<br />T;le relative numbers of hybrids and parental specie,s are no
<br />doubl; very imperfectly sho'Wl by the available data (Table I).
<br />The type of Xyrauchen uncompahgre (U.S.N.M.; No. 75992) was
<br />taken (1889) with both putative parental species, but in exactly what
<br />numbers cannot be stated, for the specimens were no doubt selected,
<br />as WHS the habit of the time, and the collection has very possibly
<br />become in part scattered. The second specimen (U.S.N.M., No.
<br />143504) was seined with one maturing female of X. texanus '~ll mm.
<br />long. No specimens of Catostomus latipinnis were preserved in
<br />the Slime series, but that species is widely dispersed throughout the
<br />
<br />SEX AND MATURITY
<br />
<br />There is no evidence of an unbalanced sex ratio among these
<br />hybrids. Of the eight known specimens, three are males, five fe-
<br />
<br />males. .
<br />The fact that the 256-mm. hybrid was wholly immature suggests
<br />reduced fecundity, very rapid gro:wth, or delayed mat~rity, any of
<br />which might well be attributed to its origin as a hybrId betwee~ a
<br />species of Catostomus and the very large Xyrauchen texanus. '1 he
<br />six large hybrids, 879-442 mm. long, ma! ~ave spawned. The fou~
<br />females appear to be spent and the emaCIatIOn of the tW? male spec-
<br />imens suggests that they too may have bred. TheIr testes are
<br />moderately enlarged.
<br />
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