Abstract Detail

Comparative Genomics/Transcriptomics

Livshultz, Tatyana [1], Smith, Chelsea R. [2], Teisher, Jordan [1].

Survey of Viridiplantae for homospermidine synthase-like genes: multiple parallel origins at multiple phylogenetic depths.

Premise: Discovery of parallelism, the independent origins of identical derived traits from identical ancestral traits, is vital to the investigation of natural selection and constraints on evolution in a comparative framework. Background: The multiple origins of homospermidine synthase, the first pathway specific gene of pyrrolizidine alkaloid biosynthesis, are outstanding examples of parallelism. Seven independent origins of pyrrolizidine alkaloids among angiosperms were each accompanied by independent evolution of homospermidine synthase (hss) via duplication and subfunctionalization of deoxyhypusine synthase (dhs), a highly conserved gene of eukaryotic primary metabolism.  Each time HSS evolved, a conserved DHS amino acid motif, IXXXN, evolved into the HSS-specific motif, VXXXD. Multiple lines of evidence suggest that loss of HSS function is accompanied by reversion to the IXXXN DHS motif or to an intermediate motif, IXXXD or VXXXN. Objectives: We conducted a broad survey of the dhs gene family to 1) determine how conserved are the identified amino acid motifs, 2) identify novel candidate hss loci, and 3) better place the origins of previously characterized hss genes in the diversification of Viridiplantae. Methods: We compiled a database of dhs-like genes using BLAST searches against 1KP and NCBI transcriptome databases and reconstructed the gene tree. Our preliminary analysis included 1712 sequences sampled from 54 of 68 seed plant orders and outgroups from all major lineages of Viridaplantae. Results: Initial results show that 1) the four previously identified amino acid motifs are highly conserved, only two species have other motifs; 2) Santalaceae, a pyrrolizidine alkaloid-producing family which has not been studied genetically, has paralogous dhs- (IXXN) and hss-like (VXXD) loci, as do multiple taxa not known to produce pyrrolizidine alkaloids; 3) the hss of Orchidaceae is the result of an ancient duplication that predates the divergence of Pandanales and Asparagales. Conclusions: Evolution of the DHS/HSS amino acid motifs is highly constrained, indicative of a high level of structural constraint on enzymatic function. Confirmation of novel candidate hss loci identified here will require functional validation. Investigation of candidate hss genes in taxa not known to produce pyrrolizidine alkaloids may reveal new pyrrolizidine alkaloid producing taxa and/or additional biosynthetic pathways which have recruited HSS. Placing known hss genes in a broader phylogenetic context shows that some duplications are more ancient than previously thought, suggestive of dynamic turnover of secondary metabolites through the evolutionary history of angiosperms.

Related Links:
2018 paper demonstrating seven parallel origins of HSS from DHS among angiosperms

1 - Academy Of Natural Sciences Of Drexel University, Department Of Botany, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103, United States
2 - Drexel University, Biodiversity Earth and Environmental Sciences, Academy Of Natural Sciences Of Drexel University, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103-1101, United States

special metabolism
Gene duplication
parallel evolution
gene trees.

Presentation Type: Oral Paper
Number: 0004
Abstract ID:522
Candidate for Awards:Margaret Menzel Award

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