. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Drosophila FMRFa Precursor M O. M Aplysia FMRFa Precursor I Figure 1. Diagonal plots (dot matrix plots, see Doolittle, 1986, for more explanation) comparing the Aplysia and Drosophila FaRP pre- cursor sequences to themselves (top, middle) and to each other (bot- each other than they are to those of the other precursor. How could such a relationship develop? The simplest explanation is that each precursor origi- nated with an independent partial duplication of an an- cestral precursor. Alternatively, as suggested by Nambu el
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Drosophila FMRFa Precursor M O. M Aplysia FMRFa Precursor I Figure 1. Diagonal plots (dot matrix plots, see Doolittle, 1986, for more explanation) comparing the Aplysia and Drosophila FaRP pre- cursor sequences to themselves (top, middle) and to each other (bot- each other than they are to those of the other precursor. How could such a relationship develop? The simplest explanation is that each precursor origi- nated with an independent partial duplication of an an- cestral precursor. Alternatively, as suggested by Nambu el at. (1988), their most recent common ancestor could have been highly duplicated and one or both of its daugh- ters suffered partial gene conversion to regularize their repeats. These possibilities are not unconnected, and both would be favored if the FMRFamide gene con- tained a so-called "hot spot"; a region of high recom- bination frequency. Of the FaRPs that do not appear to be homologous on the basis of peptide sequence, precursors are known for the pancreatic polypeptide-like peptides (Minth et ai, 1984; Leiter et ai. 1985), gammai-MSH (Nakanishi et ai. 1979), and the Aplysia L5 peptide (Shyamala et ai. 1986). The precursor encoded by the L5 gene is exem- plary: it is not detectably repetitive (Fig. 2a), and con- tains only a single stretch of sequence [RFGKR; the ter- minal dipeptide (RF) plus its amidation and cleavage sig- nals] in common with either the Aplysia or Drosophila FMRFamide precursor (Fig. 2b, c). So a comparison of the genes supports our conclusion from the peptide se- quences that this precursor is not homologous to that of FMRFamide. Conclusions The subset of FaRPs terminating in FXRFamide are probably homologs. But some members of this group may not be homologous to FMRFamide at all, while other FaRPs that lack the FXRFamide sequence will prove to be homologs once their precursors can be scruti- nized. Some FaRPs that are not in the FXRFa
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