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Abstract Detail



Macroevolution

Smith, Chelsea R. [1], Teisher, Jordan [2], Kaltenegger, Elisabeth [3], Moore, Abigail [4], Straub, Shannon [5], Livshultz, Tatyana [6].

In defense of Apocynaceae: what can we say about evolution of pyrrolizidine alkaloids from evolution of homospermidine synthase?

Since plant defenses are predicted to experience positive selection when they first evolve, patterns of selection on a gene involved in synthesis of a defensive metabolite could place the origin of the metabolite on a phylogeny. Pyrrolizidine alkaloids (PAs), defensive specialized metabolites, are rare and phylogenetically scattered in Apocynaceae. Prior research on the evolution of homospermidine synthase (HSS), the only known gene of PA biosynthesis, inferred a single origin of PAs in Apocynaceae followed by multiple losses (defense de-escalation). All Apocynaceae HSS-like genes evolved from the ubiquitous deoxyhypusine synthase (DHS) via a single duplication in the most recent common ancestor of PA-producing species. Furthermore, the characteristic HSS amino acid motif (VXXXD) evolved from the characteristic DHS motif (IXXXN) in this ancestor. In the present study, we directly tested the effect of amino acid motifs on HSS activity via mutagenesis and in vitro assays on a wild-type VXXXD HSS. We assembled an expanded dataset (142 species, 275 loci, 70% larger) to revisit ancestral sequence reconstruction in Apocynaceae HSS-like genes and test for selection to infer the number of times a highly functional HSS evolved. Some lines of evidence support defense de-escalation. Experiments confirm the importance of motifs to HSS function. (V/I)XXXN mutants have reduced HSS activity compared to VXXXD wild-type and IXXXD mutants. There was a single origin of HSS-like genes in Apocynaceae with an IXXXD motif and loss of HSS function occurred multiple times via pseudogenization and perhaps via evolution of an IXXXN motif. In contradiction to predictions from the defense de-escalation model, the VXXXD motif, present in all PA-producing species, is reconstructed as evolving two or three times independently. Branch-site models suggest that the ancestral IXXXD branch was not under positive selection while some VXXXD branches were. Furthermore, there is no evidence of relaxed selection on putatively impaired HSS-like genes (IXXXN motif) compared to those with putatively functional (V/I)XXXD motifs. Eight (8) amino acid positions experienced positive selection on branches where motifs and/or PAs may have evolved, 5 of which are predicted to affect enzyme structure and/or function. Substitutions at these sites occurred on multiple branches in the HSS-like gene tree, suggesting that HSS function may have been optimized in multiple independent lineages (defense escalation). We discuss how the complexity of genotype-phenotype maps can confound inference of phenotypic evolution from genic evolution and the prospects for greater insight via evolutionary reconstruction of entire biosynthetic pathways.


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Related Links:
Previous work: Livshultz T, Kaltenegger E, Straub SCK, Weitemier K, Hirsch E, Koval K, Mema L, Liston A. 2018. Evolution of pyrrolizidine alkaloid biosynthesis in Apocynaceae: revisiting the defence de-escalation hypothesis. New Phytol 218:762-773.


1 - Academy of Natural Sciences of Drexel University, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103, United States
2 - The Academy Of Natural Sciences Of Drexel University, Botany, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103, United States
3 - Christian-Albrechts-Universit├Ąt zu Kiel, Botanisches Institut und Botanischer Garten
4 - Ohio University, Environmental and Plant Biology, 22 Richland Ave, Athens, OH, 45701, United States
5 - Hobart And William Smith Colleges, Department Of Biology, 300 Pulteney St., Geneva, NY, 14456, United States
6 - Academy Of Natural Sciences Of Drexel University, Department Of Botany, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103, United States

Keywords:
Apocynaceae
Gene duplication
specialized metabolism
pyrrolizidine alkaloid
tests for selection.

Presentation Type: Oral Paper
Session: MACROII, Macroevolution II
Location: Virtual/Virtual
Date: Thursday, July 22nd, 2021
Time: 1:15 PM(EDT)
Number: MACROII004
Abstract ID:599
Candidate for Awards:None


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