Transgenic study of parallelism in plant morphological evolution

Abstract

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 ...

Systematics and phylogeny of the Brassicaceae (Cruciferae): an overview

“…For example, much taxonomic emphasis has been placed on the arrangement of flowers in racemes or on solitary pedicels originating from the basal rosette. We now know that a few genes, including LEAFY, control the development of rosette instead of raceme flowering in the Brassicaceae (Shu et al 2000, Baum 2002, Yoon and Baum 2004, and several genera (e.g. Leavenworthia, Selenia) can have both types of flowering on the same plant.…”

Genetic Framework for Flattened Leaf Blade Formation in Unifacial Leaves ofJuncus prismatocarpus

“…Although the independent evolution of similar morphological traits is widespread, the underlying mechanisms are not fully understood (Gould, 2002;West-Eberhard, 2003;Yoon and Baum, 2004). Flattened leaves have independently evolved in bifacial and unifacial leaves, probably for efficient light capture.…”

Convergent evolution of caffeine in plants by co-option of exapted ancestral enzymes

“…convergent evolution | caffeine biosynthesis | enzyme evolution | paleomolecular biology C onvergent evolution has resulted in the independent origins of many traits dispersed throughout the tree of life. Whereas some convergent traits are known to be generated via similar developmental or biochemical pathways, others arise from different paths (1)(2)(3)(4)(5). Likewise, similar (orthologous) or different (paralogous or even unrelated) genes may encode for the regulatory or structural proteins composing the components of pathways that build convergent traits (6)(7)(8)(9)(10).…”