Synteny and Collinearity in Plant Genomes

Bias in plant gene content following different sorts of duplication: tandem, whole-genome, segmental, or by transposition

Each mode of gene duplication (tandem, tetraploid, segmental, transpositional) retains genes in a biased manner. A reciprocal relationship exists between plant genes retained postpaleotetraploidy versus genes retained after an ancient tandem duplication. Among the models (C, neofunctionalization, balanced gene drive) and ideas that might explain this relationship, only balanced gene drive predicts reciprocity. The gene balance hypothesis explains that more "connected" genes--by protein-protein interactions in a heteromer, for example--are less likely to be retained as a tandem or transposed duplicate and are more likely to be retained postpaleotetraploidy; otherwise, selectively negative dosage effects are created. Biased duplicate retention is an instant and neutral by-product, a spandrel, of purifying selection. Balanced gene drive expanded plant gene families, including those encoding proteasomal proteins, protein kinases, motors, and transcription factors, with each paleotetraploidy, which could explain trends involving complexity. Balanced gene drive is a saltation mechanism in the mutationist tradition.

Finding and comparing syntenic regions among Arabidopsis and the outgroups papaya, poplar, and grape: CoGe with rosids

In addition to the genomes of Arabidopsis (Arabidopsis thaliana) and poplar (Populus trichocarpa), two near-complete rosid genome sequences, grape (Vitis vinifera) and papaya (Carica papaya), have been recently released. The phylogenetic relationship among these four genomes and the placement of their three independent, fractionated tetraploidies sum to a powerful comparative genomic system. CoGe, a platform of multiple whole or near-complete genome sequences, provides an integrative Web-based system to find and align syntenic chromosomal regions and visualize the output in an intuitive and interactive manner. CoGe has been customized to specifically support comparisons among the rosids. Crucial facts and definitions are presented to clearly describe the sorts of biological questions that might be answered in part using CoGe, including patterns of DNA conservation, accuracy of annotation, transposability of individual genes, subfunctionalization and/or fractionation of syntenic gene sets, and conserved noncoding sequence content. This précis of an online tutorial, CoGe with Rosids (http://tinyurl.com/4a23pk), presents sample results graphically.

Dynamic evolution of oryza genomes is revealed by comparative genomic analysis of a genus-wide vertical data set

Oryza (23 species; 10 genome types) contains the world's most important food crop - rice. Although the rice genome serves as an essential tool for biological research, little is known about the evolution of the other Oryza genome types. They contain a historical record of genomic changes that led to diversification of this genus around the world as well as an untapped reservoir of agriculturally important traits. To investigate the evolution of the collective Oryza genome, we sequenced and compared nine orthologous genomic regions encompassing the Adh1-Adh2 genes (from six diploid genome types) with the rice reference sequence. Our analysis revealed the architectural complexities and dynamic evolution of this region that have occurred over the past approximately 15 million years. Of the 46 intact genes and four pseudogenes in the japonica genome, 38 (76%) fell into eight multigene families. Analysis of the evolutionary history of each family revealed independent and lineage-specific gain and loss of gene family members as frequent causes of synteny disruption. Transposable elements were shown to mediate massive replacement of intergenic space (>95%), gene disruption, and gene/gene fragment movement. Three cases of long-range structural variation (inversions/deletions) spanning several hundred kilobases were identified that contributed significantly to genome diversification.

Unraveling ancient hexaploidy through multiply-aligned angiosperm gene maps

Large-scale (segmental or whole) genome duplication has been recurring in angiosperm evolution. Subsequent gene loss and rearrangements further affect gene copy numbers and fractionate ancestral gene linkages across multiple chromosomes. The fragmented "multiple-to-multiple" correspondences resulting from this distinguishing feature of angiosperm evolution complicates comparative genomic studies. Using a robust computational framework that combines information from multiple orthologous and duplicated regions to construct local syntenic networks, we show that a shared ancient hexaploidy event (or perhaps two roughly concurrent genome fusions) can be inferred based on the sequences from several divergent plant genomes. This "paleo-hexaploidy" clearly preceded the rosid-asterid split, but it remains equivocal whether it also affected monocots. The model resulting from our multi-alignments lays the foundation for approximating the number and arrangement of genes in the last universal common ancestor of angiosperms. Comparative analysis of inferred homologous genes derived from this model shows patterns of preferential gene retention or loss after polyploidy and reveals large variability of nucleotide substitution rates among plant nuclear genomes.

A gender-specific retinoblastoma-related protein in Volvox carteri implies a role for the retinoblastoma protein family in sexual development

Here, we describe the cloning and characterization of RETINOBLASTOMA-RELATED PROTEIN1 (RBR1) from the green alga Volvox carteri. RBR1 expression increases substantially during embryogenesis and in response to the sex-inducer glycoprotein, but it decreases significantly under heat stress. While RBR1 is expressed in gonidia (asexual reproductive cells) and embryos, the largest proportion of RBR1 mRNA is found in parental somatic cells. The presence of 4 splice variants and 15 potential cyclin-dependent kinase phosphorylation sites suggests that RBR1 is subject to control at the posttranscriptional and posttranslational levels. Surprisingly, RBR1 is a gender-specific gene, mapping exclusively to the female mating-type locus. A procedure for stable nuclear transformation of males was established to generate RBR1-expressing males. These transformants exhibit enlarged reproductive cells, altered growth characteristics, and a prolonged embryogenesis. The results suggest that a functionally related analog of RBR1 exists in males. The reason for the divergent evolution of RBRs in females and males appears to be based on sexual development: males and females respond to the same sex-inducer with different cleavage programs and substantial differences in cellular differentiation. Thus, the gender-specific presence of RBR1 provides evidence for an additional, novel role for retinoblastoma family proteins in sexual development.

Multiple paleopolyploidizations during the evolution of the Compositae reveal parallel patterns of duplicate gene retention after millions of years

Of the approximately 250,000 species of flowering plants, nearly one in ten are members of the Compositae (Asteraceae), a diverse family found in almost every habitat on all continents except Antarctica. With an origin in the mid Eocene, the Compositae is also a relatively young family with remarkable diversifications during the last 40 My. Previous cytologic and systematic investigations suggested that paleopolyploidy may have occurred in at least one Compositae lineage, but a recent analysis of genomic data was equivocal. We tested for evidence of paleopolyploidy in the evolutionary history of the family using recently available expressed sequence tag (EST) data from the Compositae Genome Project. Combined with data available on GenBank, we analyzed nearly 1 million ESTs from 18 species representing seven genera and four tribes. Our analyses revealed at least three ancient whole-genome duplications in the Compositae-a paleopolyploidization shared by all analyzed taxa and placed near the origin of the family just prior to the rapid radiation of its tribes and independent genome duplications near the base of the tribes Mutisieae and Heliantheae. These results are consistent with previous research implicating paleopolyploidy in the evolution and diversification of the Heliantheae. Further, we observed parallel retention of duplicate genes from the basal Compositae genome duplication across all tribes, despite divergence times of 33-38 My among these lineages. This pattern of retention was also repeated for the paleologs from the Heliantheae duplication. Intriguingly, the categories of genes retained in duplicate were substantially different from those in Arabidopsis. In particular, we found that genes annotated to structural components or cellular organization Gene Ontology categories were significantly enriched among paleologs, whereas genes associated with transcription and other regulatory functions were significantly underrepresented. Our results suggest that paleopolyploidy can yield strikingly consistent signatures of gene retention in plant genomes despite extensive lineage radiations and recurrent genome duplications but that these patterns vary substantially among higher taxonomic categories.