Linkage Relationships between the Homologous Malate Dehydrogenase Loci in Teleosts.

ENE duplication is an important evolutionary mechanism and may have led to large increases in genome size and diversity early in vertebrate evolution (OHNO 1970). The role of duplicate genes may be further elucidated by biochemical and genetic analyses of multigenic isozyme system in modern teleosts. Gene duplications are generally detected by the biochemical multiplicity of gene products. Apparent duplications have been found in vertebrates for a number of enzyme systems (SHAW 1969; MARKERT and WHITT 1968). Extensive gene duplication has occurred in salmonid and cyprinid fish as a result of tetraploid erolution (OHNO 1970). Specific duplications have occurred in most teleosts. Examples of teleost loci arising from duplication are the lactate dehydrogenase E and B loci (WHITT 1969, 1970; WHITT, CHILDERS and WHEAT 1971) and the supernatant malate dehydrogenase A and B (MDH-A and MDH-B) loci (KARIG and WILSON 1971). The inheritance of MDH-B has been studied in interspecific bass hybrids (WHEAT and WHITT 1971; WHEAT et al. 1971). In addition, BAILEY et al. (1969, 1970) have biochemically analyzed both the supernatant MDH-A and MDH-B isozymes of salmon. These MDH isozymes are very similar in kinetic properties, amino acid composition, and immunochemical cross-reactivity (BAILEY et al. 1970). The good correlation between immunological similarity and sequence homology established for other enzymes (PRAGER and WILSON 1971) suggests extraordinary homology between MDH-A and -B. Furthermore, the structural similarity of the supernatant MDH-A and -B polypeptides in interspecific centrarchid hybrids is confirmed by the random assembly of these subunits into dimers both in uiuo and in vitro (WHEAT and WHITT 1971; WHEAT et al. 1971 ) . The present study is one of the first analyses of linkage relationships between homologous loci specifying such closely related enzymes. Interspecific hybrids between bluegills (B) (Lepomis macrochirus Rafinesque)