Endemic Whitefishes of Bear Lake, Utah-Idaho: A Problem in Systematics
Abstract
The systematic status of whitefishes endemic to Bear Lake, Utah- Idaho, has remained tenuous since t heir original description. Clarification of this problem was the major objective of the present study. The general approach was an integrated one, including examination of morphological, biochemical and ecological parameters; artificial hybrids were produced and compared with questionable groups from the natural population.
Morphological analysis revealed five forms of Bear Lake whitefishes. Prosopium gemmiferum (Bonneville cisco) and !· abyssicola (Bear Lake whitefish) were well differentiated from other forms and were treated as originally described. The P. spilonotus (Bonneville whitefish) group, however, was found to be made up of two morphologically distinct populationa, referred to as P. spilonotus (small form) and !· spilonotus (large form). The fifth group referred to as!· gemmiferum-like (represented by only five specimens) was intermediate between!· gemmiferum and either P. spilonotus (small form) or !· abyssicola and was hypothesized to be of hybrid origin. Mul tiple discriminant function analysis of the four major groups and ~· williamson! (mountain whitefish) (Logan River) confirmed morphological differentiation between forms .
Hybridization studies among Bear Lake Prosopium and P. williamson! involved 50 homo- and hetero specific crosses (17 combinations). Of 12 experimental hybrid combinations attempted , all those involving simul taneously ripe specimens of two groups (five crosses) showed maximum fertilization success equalling that of pure crosses. No evidence that interspecific crosses are l ess successful than conspecific crosses, with the possible exception of ~· williamson! ~ x ~· gemmiferum d (W x G), was obtained. Culture methods were developed and morphological comparisons made.
Origin of ~· gemmiferum-like hybrids in the lake population was not consistently explained by morphological comparison of known ~· spilonotus (small form) ~ x ~· gemmiferum d (S x G) hybrids or ~· abyssicola 'i' x ~· gemmiferum d (A x G) hybrids; morphometric characters were more like S x G hybrids while meristic characters were more closely associated with A x G hybrids. Based on evidence available, no definitive statement could be made concerning the origin of ~· gemmiferum-like hybrids except that they are hybrids among combina tions of~· gemmiferum and either P. spilonotus (small form) or ~· abyssicola. No known hybrid explained the origin of either group of ~· spilonotus .
Electrophoretic analysis of general proteins and several enzyme systems of various tissues showed much similarity among Bear Lake Prosopium; only ~· williamsoni was totally unique. Biochemical evidence did not support or refute separate consideration of the two forms of ~· spilonotus but did establish that neither were phenotypic variants of ~· williamson!.
Ecological characteristics of Bear Lake Prosopium revealed important distinctions between forms. Growth histories of ~· abyssicola, ~· spilonotus (small form) and ~· spilonotus (large form) showed pronounced differences. Distinct differences in growth and in age and size at maturity of forms of ~· spilonotus provided further evidence supporting their separate consideration. Spatial overlap of spawning activities was marked between forms of ~· spilonotus and ~· gemmiferum; ~· abyssicola was well separated spatially. Temporally, slight overlap was observed between ripe females of one group and ripe males of the succeeding group · to spawn. The only observation of the simultaneous occurrence of ripe females of two forms was between ~· spilonotus (large form) and P. spilonotus (small form); in this instance, the number of ripe females of each form was extremely small. No evidence of mass hybridization among forms was observed. A combination of temporal, spatial and ethological premating isolating mechanisms are thought to be important in reproductive isolation of Bear Lake whitefishes while postmating mechanisms are nonfunctional with the possible exception of hybrid sterility.
Morphological and ecological analyses, combined with results of experimental hybridization, provided abundant evidence supporting separate recognition of the two forms of ~· spilonotus. Karyotypes of P. gemmiferum, ~· abyssicola and ~· spilonotus (small form) have been determined (Booke, 1974) and are unique for each species. If the karyotype of P. spilonotus (large form) is found to also be unique, there should be no question that the two forms of ~· spilonotus represent distinct species. Final clarification of the taxonomic status of these forms will not come until karyotype data is available; however, based upon present evidence, tentative recognition of a new species is recommended.