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<br />228
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<br />II ubbs and .111 iller
<br />
<br />tionship in families for which the taxonomic analysis has not yet
<br />cleaned tribes). This close correlation of hybridization 'with classifi-
<br />cal;ion raises the question whether the Cyprinidae of eastern and of
<br />western North America hybridize more heterogenetieally-even
<br />becween subfamilies according to current classifications--or whether
<br />th,~ groups of genera involved may not have been assigned too high a
<br />rallking. .
<br />It would be flagrantly inconsistent with taxonomic systems, how-
<br />eVI~r, to reduce to subspecific status the many species in such fami-
<br />lie:; as the Catostomidae and Cyprinidae that do hybridize in nature,
<br />ev[~n if it could be shown that the hybrids are to some degree fertile.
<br />Wllether Xyrauchen or Chasmistes or both (or Pantosteus) should be
<br />united with Catostomus must remain a matter of judgment or a.u-
<br />thority, but we feel no need to synonymize these genera because they
<br />do hybridize with some frequency. Crossability we regard as only
<br />one of many systematic criteria, all of which need to be jointly con-
<br />sidered in taxonomic ranking.
<br />When a poorly known and troublesome nominal species such as
<br />Xl/Tal/chen uncompahgre is indicated as having been based on a spo-
<br />radic interspecific hybrid, it should, of course, be deleted from the
<br />system. Such clarifications constitute one of the major contribu-
<br />tioJls from the study of natural hybridization. Since the name
<br />unl:ompahgre was not based on a systematic entity, it should be
<br />regarded as unavailable.
<br />The newly discovered hybrids again emphasize the generalization
<br />that, throughout the teleost series, hybridization between species is
<br />much commoner among freshwater than among marine fishes. We
<br />think that this interesting but clear-cut contrast may be attribut.ed,
<br />in part at least, to the unstable and temporary nature of freshwater
<br />habitats and to the increased chance for the meeting of ova and
<br />spermatozoa in stream currents. That the hybridization of Xyrau-
<br />chen and of Chasrnistes with Catostomus may be attributable to such
<br />chlLllce mixing of the sex products on stream riffles seems very likely,
<br />in view of the large discrepancy in size that is typical of these genera
<br />and in view of the complex courtship and pairing that characterizes
<br />the catostomids (Reighard, 1920).
<br />Enough is known of the habitats and the breeding seasons of
<br />Calostomus and Xyrauchen (and of Chasmistes) to make it appear
<br />
<br />Hybridization between Catostomus and Xyrauchen
<br />
<br />229
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<br />virtually certain that these genera do spawniIi cl~sed prb:idrrlity; ~:
<br />Some humpback suckers presumably spawn in small streams (where
<br />species of Catostomus also breed), or at least did so under less modi-
<br />fied, stream conditions. It,is well known"that:th~,jspecieS;()f~gafu:;;"'~
<br />storn.us,. spawn .,in ,the, spring., It has, heenJourid;!that"X:jte1afiiiS"
<br />~spawns' from Fehrua~y. well into April in the lower.part~of;f;he,ii;' i
<br />b510'rado'River'.systeDi (local testimony obtained hy the "jti~ior"
<br />author and observations by Douglas, 1952, and by us). This species
<br />spawns later in the year, though presumably at a comparable season,
<br />in the upper Co!orado system. Females collected.~her~ih;\Mfit~er~
<br />~,'not yet fully rIpe and Jordan (1891, p. 25),reportedthat:C~;,7att-
<br />;":tPinn, is and X yrmlbhen~we,r, e both said to spawn in ,the suiliiHer in
<br />tRibAnimas, Colorado;
<br />\"-Ariolher factor that has probably contributed to the hybridiza-
<br />tion of the two genera, increasingly so in recent years, is the abun-
<br />dance of Catostomus and the relative scarcity of Xyrauchen. Marked
<br />differences in the numbers of the parental species appear to have in-
<br />creased the frequency of hybridization between other species of
<br />the Catostomidae (Hubbs and Hubbs, 1947, p. 153), and of other
<br />families, such I1s the Cyprinodontidae (Hubbs, Walker, and Johnson,
<br />1943). Individuals of the rare species are presumably often at-
<br />tracted to the spawning grounds of the common species, where the
<br />chance of hybridization is increased, as through the chance meeting
<br />of eggs and sperm. When both species are.coIIl~6tl;;miribr'dif-
<br />1j~~~:~~ces.ii~i're,ac~ion~\Jorbeh~".ior presumably. keepj;,the spa ~ing':,'"
<br />:',~:~~?sa,part~<:,,~,;""'."'''MH..~'-'"'
<br />
<br />SUMMARY
<br />
<br />The genera of the freshwater teleost family Catostomidae that
<br />hybridize in nature include Chasmistes and Xyrauchen as well as
<br />Catostomus and Pantosteus. Two new combinations are described,
<br />both between species endemic to the Colorado River system: (1)
<br />Catostomus latipinnis X Xyrauchen texanus (eight specimens, in-
<br />cluding the holotype of the nominal species X. uncompahgre, now
<br />deleted from the system), and (2) Catostomus insignis X Xyrauchen
<br />texanus (six specimens, in one collection). These hybrids have
<br />occurred sporadically in waters where the parental species have un-
<br />doubtedlyboth bred. Greatly reduced numbers of one parental
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