TANK, DAVID C.1,2*, XIAO-QUAN WANG3, and TAO SANG2. 1Department of Botany, University of Washington, Box 355325, Seattle, WA 98195; 2Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824; 3Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China. - Recent transfers of cinnamyl alcohol dehydrogenase genes between conifers diverged 200 million years ago.
The nuclear genome of conifers is large and organized in complex gene
families, and due primarily to a lack of comparative studies of
protein coding nuclear genes across conifer families, very little is
known about its evolutionary dynamics. In this study, we investigated
the evolution of a low-copy nuclear gene encoding cinnamyl alcohol
dehydrogenase (CAD), a key enzyme in the lignin biosynthetic pathway,
in Pinaceae and Taxodiaceae. A portion of the CAD gene, including
three exons and four introns, was PCR-amplified, cloned, and sequenced
from all eleven genera of Pinaceae and five sampled genera from
Taxodiaceae. Sequences of 627 nucleotides of the three exons were
aligned among the sampled conifer species, as well as angiosperm
species used as outgroups. Phylogenetic analysis of the aligned exon
sequences yielded 862 most parsimonious trees, of which the strict
consensus topology of the ingroup taxa is identical to that of the
maximum likelihood tree. Surprisingly, a clade, which contains members
of Taxodiaceae and species of Abies (Pinaceae), is nested
within Pinaceae and has extremely short internal branches. The average
synonymous and nonsynonymous divergences within this clade are 0.0149
+ 0.0058 and 0.0074 + 0.0019, respectively. Given that Pinaceae and
Taxodiaceae diverged as early as the Triassic Era, it is striking that
such similar CAD sequences were found. Herein, three hypotheses
explaining the remarkably similar CAD sequences are examined, DNA
contamination, extremely slow rates of sequence divergence, and recent
lateral gene transfer. Ultimately, we conclude that it is most likely
that the CAD genes were recently transferred from Abies to
members of Taxodiaceae. The lateral transfer of functionally important
structural genes between reproductively isolated plants opens a new
avenue for the exchange of genetic material, and may provide new
insights into conifer evolution.
Key words: cinnamyl alcohol dehydrogenase, lateral gene transfer, Pinaceae, Taxodiaceae