THE POTENTIAL FOR HYBRIDIZATION IN FRESH-WATER COPEPODS

The importance of hybridization and hybrid zones in pelagic systems is largely unknown, in part because planktonic species are generally ass umed to be reproductively isolated. However, lakes in their entirety r epresent potential hybrid zones throughout which sympatric species may mate and hybridize. Recent field evidence for the existence of interm ediate phenotypes together with behavioral observations of heterospeci fic matings suggest that hybridization may be much more common than pr eviously thought in these systems. This study examines the potential f or hybridization of two related copepod species, Diaptomus minutus and D. pygmaeus, that co-occur in lakes throughout the northeastern Unite d States. Field and experimental laboratory data were collected to: (1 ) compare the spatial and temporal occurrence of these two congeners i n a single lake; (2) quantify the extent to which mating errors occur in situ; (3) examine the species specific mating cues that potentially affect the probability of hybridization; and (4) determine the potent ial for gametic compatibility and hybrid viability of these two specie s. D. minutus and D. pygmaeus are spatially and temporally sympatric, and the timing of their reproductive activity can be coincident in sit u. One-directional mating errors occur frequently in both the laborato ry and the field (e.g., upto 70% of D. minutus females in situ may car ry spermatophores from D. pygmaeus males). Very low but successful pro duction of hybrids also occurs, demonstrating that their gametes are i ndeed compatible and their hybrids are sometimes viable. These results underscore the potential for hybridization to play a greater role in speciation and contribute more to phenotypic diversity in aquatic crus tacean communities than previously shown.