Many filamentous cyanobacteria grow as multicellular organisms that show a developmental pattern of single nitrogen-fixing heterocysts separated by approximately 10 vegetative cells. Overexpression of a 54-base-pair gene, patS, blocked heterocyst differentiation in Anabaena sp. strain PCC 7120. A patS null mutant showed an increased frequency of heterocysts and an abnormal pattern. Expression of a patS-gfp reporter was localized in developing proheterocysts. The addition of a synthetic peptide corresponding to the last five amino acids of PatS inhibited heterocyst development. PatS appears to control heterocyst pattern formation through intercellular signaling mechanisms.

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Developmental constraint is indicated when one finds that similar genetic mechanisms are responsible for independent origins of the same derived phenotype. We studied three independent origins of rosette flowering within the mustard family and attempted to evaluate the extent to which the same mechanisms were involved in each transition from the ancestral phenotype, inflorescence flowering. We used transformation to move a candidate gene, LFY, and its cis-regulatory sequences from rosette-flowering species into an inflorescence-flowering recipient, Arabidopsis thaliana, in place of its endogenous LFY gene. The transgenic phenotypes of experimental and control lines (containing an A. thaliana LFY transgene) and the expression driven by the cis-regulatory sequences show that changes at the LFY locus might have contributed to the evolution of rosette flowering in two of the three lineages. In the third case, changes upstream of LFY are implicated. Our data suggest that changes in a single developmental regulatory program were involved in multiple origins of the same derived trait but that the specific genetic changes were different in each case. P arallelism refers to the independent evolution of the same derived trait via the same developmental changes, whereas convergence refers to superficially similar traits that have a distinct developmental basis (1). The most compelling reason to distinguish these two phenomena is because they serve to document different phenomena. Convergence provides evidence of the efficacy of natural selection, whereas the occurrence of parallelism shows that the path of evolution is constrained to certain channels determined by the structure of developmental programs (2, 3).Strict parallelism, where the identical mutation occurs repeatedly, is documented in cases of biochemical adaptation to toxins (4-6). When dealing with morphology, however, the concept of parallelism is usually relaxed to include cases of different mutations to the same target locus and even changes to different genes within the same developmental pathway. Under this broader definition, parallelism can be elucidated by using comparative gene expression data (7-10). In such studies, however, there is always the concern that changes in gene expression might be far downstream of the ultimate genetic causes of morphological homoplasy. A few studies have used classical genetic data to study homoplasy (11)(12)(13)(14), and some studies have provided strong evidence in favor of parallelism (13,14). Here we use a transgenic approach that is not limited to cases in which study species are crossable (15). Although interspecies transformation has been used to elucidate male song evolution in Drosophila (16) and the origin of self-compatibility in Arabidopsis thaliana (17), it has not to our knowledge been applied to the problem of parallel evolution.Most species of Brassicaceae, including A. thaliana, bear flowers in an inflorescence, an elongated portion of stem on which the leaves that would otherwise subtend the flowers are ...