Single fast muscle fibres in the tropical fish Oreochromis andersonii were found to contain two myosin light chains (LC1s; LC1f1* or LC1f2*). Breeding experiments confirmed that the different LC1s were of allelic origin and their inheritance patterns conformed to Mendelian expectations (1:2:1). The LC1s differed in apparent relative molecular mass by 800­900. No other differences in myosin subunits were found between the LC1 genotypes. The molar ratios of LC3:LC1(total) in the fast muscle of O. andersonii homozygous for LC1f1* or LC1f2* and heterozygous for both alleles were 2.0:1, 2.1:1 and 2.2:1, respectively, as determined by capillary electrophoresis. The maximum contraction velocity (Vmax) of single skinned muscle fibres was determined at 20 °C by the slack-test method. Vmax values (fibre lengths s-1) for fast muscle fibres from O. andersonii which were homozygous for either LC1f2* or LC1f1* were 5.3 and 3.3, respectively, compared with 3.8 when both alleles were present. Crosses between Oreochromis niloticus and O. andersonii produced F1 hybrids which were heterozygous for either LC1n/LC1f1* or LC1n/LC1f2*, where LC1n is the myosin light chain for O. niloticus. The distribution of myosin light chain genotypes in hybrid offspring was not significantly different from the expected Mendelian 1:1 ratio (47 %: 53 %). The Vmax (fibre lengths s-1) of muscle fibres containing LC1f2* from hybrid Oreochromis was 4.3 compared with 3.1 for the LC1f1* genotype. The results are consistent with a functionally significant allelic variation in myosin LC1 in fast muscle fibres from O. andersonii which is also expressed in hybrid genotypes.
Thermal tolerance and the respiratory properties of isolated red muscle mitochondria were investigated in Oreochromis alcalicus grahami from the alkaline hot-springs, Lake Magadi, Kenya. Populations of O. a. grahami were resident in pools at 42.8 °C and migrated into water reaching temperatures of 44.8 °C for short periods. The maximum respiration rates of mitochondria with pyruvate as substrate were 217 and 284 natom O mg-1 mitochondrial protein min-1 at 37 °C and 42 °C, respectively (Q10=1.71). Fatty acyl carnitines (chain lengths C8, C12 and C16), malate and glutamate were oxidised at 70­80 % of the rate for pyruvate. In order to assess evolutionary temperature adaptation of maximum mitochondrial oxidative capacities, the rates of pyruvate and palmitoyl carnitine utilisation in red muscle mitochondria were measured from species living at other temperatures: Notothenia coriiceps from Antarctica (-1.5 to +1 °C); summer-caught Myoxocephalus scorpius from the North Sea (10­15 °C); and Oreochromis andersoni from African lakes and rivers (22­30 °C). State 3 respiration rates had Q10 values in the range 1.8­2.7. At the lower lethal temperature of O. andersoni (12.5 °C), isolated mitochondria utilised pyruvate at a similar rate to mitochondria from N. coriiceps at 2.5 °C (30 natom O mg-1 mitochondrial protein min-1). Rates of pyruvate oxidation by mitochondria from M. scorpius and N. coriiceps were similar and were higher at a given temperature than for O. andersoni. At their normal body temperature (-1.2 °C), mitochondria from the Antarctic fish oxidised pyruvate at 5.5 % and palmitoyl-dl-carnitine at 8.8 % of the rates of mitochondria from the hot-spring species at 42 °C. The results indicate only modest evolutionary adjustments in the maximal rates of mitochondrial respiration in fish living at different temperatures.