Comparative biochemistry, immunology and cytogenetics

Abstract

Discussion of the phylogenetic status of the cyclostomes has so far been very largely confined to morphological considerations, with little regard to comparative biochemistry and physiology. To a certain extent, the responsibility for this situation can be laid at the door of the biochemists themselves who have often shown some reluctance to follow the logic of their own data and have adopted an attitude reflected in the words of Florkin (1966) that ‘we must adopt the methodological rule to be led by knowledge of phylogeny in our search for biochemical evolution, rather than be brought by biochemistry to the discovery of new aspects of phylogeny’. We must of course, recognize that when taken in isolation, the distribution of a particular molecule within or between groups of animals may be of little phylogenetic significance, although such information may become more important where we have more detailed information on biosynthetic pathways and enzyme systems. An example is the erratic distribution of creatine even within animals belonging to the same taxonomic group (Watts and Watts, 1974). This nitrogenous compound has been found in the tissues of cephalochordates, myxinoids and lampreys, although only the latter animals possess the necessary enzymes to synthesize it. In the other two chordate groups, as in some invertebrates, its presence may be due to the ability of these animals, either to recover it from their food or to absorb it from their aquatic environment. However, as Barrington (1974) has pointed out, even where we have information on biosynthetic pathways, their presence in different organisms or groups of animals may be open to several alternative explanations. While it may be evidence of a common ancestry, it is also possible that it is a result of parallel evolution or reflects biosynthetic abilities that are widely distributed among diverse animal groups, as exemplified in the occurrence of steroids or polypeptides such as insulin in a broad spectrum of animal groups.