High conservation among sequences encoding type-V thionins in wheat and Aegilops.

Thionins are Cys-rich plant polypeptides of about 5 kD that are synthesized as larger precursors, which include a typical signal peptide and a posttranslationally processed, C-terminal peptide, and are thought to be involved in plant defense (García-Olmedo et al., 1989, 1991, 1992). We have previously reported on a nove1 neutral thionin type in wheat (Triticum aestivum L.), designated type V, which has diverged extensively from type I by a process of accelerated evolution specially affecting the mature protein domain of the precursor (Castagnaro et al., 1992). It was further shown that type-V genes were located within a few kilobases of type-I genes in the long arms of group-1 chromosomes in the three genomes (ABD) of allohexaploid wheat and that both types were simultaneously expressed in endosperm (Castagnaro et al., 1992). During the transition between the two types, the nonsynonymous substitution rate in the mature protein domain of the precursor was equal to the synonymous rate, a rather singular situation, as it is now well known that nonsynonymous nucleotide substitutions, which do cause amino acid changes, occur at rates that are between 2 and 20 times slower than those of synonymous substitutions (Li et al., 1984; Graur, 1985; Graur and Li, 1988). Accelerated changes at functionally relevant sequences have been explained in terms of positive Darwinian selection (Hill and Hastie, 1987) or, alternatively, in connection with the neutral theory of evolution (Graur, 1985; Graur and Li, 1988). According to the second alternative, the relative mutability of a given sequence depends on the proportion of certain amino acid residues rather than on their specific positions, and can be predicted using an empirical index of mutability, Im (Graur, 1985). Because the Im value for the only known nucleotide sequence of type V (Im = 2.693) was greater than those found for type-I sequences (Im < 0.946), the observed divergence was compatible with a neutral hypothesis (Castagnaro et al., 1992). We have now determined two additional type-V sequences, which allow us to conclude that contrary to what would be expected from their higher Im values, evolution among type-V thionins is as slow as among those of type I.