LDH-B genotype-specific hatching times of Fundulus heteroclitus embryos

The evolutionary significance of protein polymorphisms has long been debated. Exponents of the balanced theory advocate that selection operates to maintain polymorphisms, while those of the neoclassical school argue that most genetic variability is selectively neutral 1 . As the ‘neutralist’ hypothesis implies that allelic isozymes are functionally equivalent, some investigators have examined the biochemistry of protein polymorphisms 2–15 , while others have concentrated on life history correlates 16–23 . Few studies, however, have established that in vitro functional differences are reflected at the whole organism level 5,21–23 , yet this is a critical link in understanding the significance of protein polymorphisms. We have studied the effects of the kinetically different lactate dehydrogenase-B (LDH-B) allelic isozymes on the rate of development and physiological performance of the fish, Fundulus heteroclitus , and report here that hatching time is highly correlated with LDH-B genotype: LDH-B a B a individuals hatch before LDH-B b B b fish while heterozygote ( LDH-B a B b ) hatching is intermediate. The basis for this phenomenon may be a differential ability to deliver oxygen to respiring tissues. As precisely timed hatching is critical to the survival of Fundulus 24 , such differences in hatching time between LDH-B genotypes may be an important component of the species' evolutionary strategy.