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Developmental and Structural Section

Lindqvist, Charlotte [3], Teeri, Teemu H. [2], Jobson, Richard W. [1], Albert, Victor A. [3].

Codon bias, synonymous rate difference, and functional divergence in duplicate PISTILLATA-like genes of Hawaiian Clermontia (Lobelioideae: Campanulaceae).

The Hawaiian endemic genus Clermontia comprises 22 species of paleotetraploid origin (n=14), ca. 2/3 of which are characterized by a homeotic phenotype in which sepals are replaced by a second whorl of petals. Phylogenetic analysis of nrDNA 5S-NTS sequences (GenBank PopSet 11386030) revealed poor genetic divergence, but resolved a single origin of this double-corolla trait by ca. 3.5 My ago, followed by 2 or more reversals. We have previously shown that a Clermontia homologue of the MADS box gene PISTILLATA (PI) expresses ectopically (i.e., heterochronically) in the first whorl of double-corolla flower buds. Using this data to guide a candidate gene strategy, we isolated full-length PI and APETALA3 (AP3) homologues from flower bud mRNA using RT-PCR. Two PI-like and 3 AP3-like copies were cloned and sequenced. The AP3-like copies, which may represent alleles, were less divergent than the PI-like copies. Molecular evolutionary analysis of the PI-like copies (hereafter Cler.PI.A and Cler.PI.B) relative to the Gerbera (Asteraceae) PI homologue GGLO1 revealed a significant (P=0.03) difference between the Clermontia copies in synonymous substitution rates (higher in Cler.PI.A), but no significant difference for nonsynonymous rates. This finding was mirrored by greater codon bias in Cler.PI.A than Cler.PI.B. These results have implications for differential expression of the duplicates, whether they derive from polyploidization or otherwise. After gene duplication, the duplicated PI-like genes could have been functionally redundant, resulting in weakened selection in one copy (Cler.PI.B). As such, the Cler.PI.B gene may have transiently experienced reduced PI function, possibly via cis-regulatory mutations. This may then have caused a lower level of expression in Cler.PI.B, resulting in a period of neutral evolution at synonymous sites. We construct a model whereby the Cler.PI.B gene may have been rescued from neutrality by functional innovation, i.e., heterochronic expression of PI function in the first whorl.


1 - Cornell University, L.H. Bailey Hortorium, Department of Plant Biology, Ithaca, New York, 14853, U.S.A.
2 - University of Helsinki, Department of Applied Biology, P.O. Box 27, Helsinki, FIN-00014, Finland
3 - University of Oslo, Botanical Garden, Natural History Museums and Botanical Garden, P.O. Box 1172 Blindern, Oslo, NO-0318, Norway

Keywords:
MADS box genes
heterochrony
gene silencing
gene evolution
B-function
PISTILLATA
APETALA3
Hawaii
Campanulaceae
Lobelioideae.

Presentation Type: Paper
Session: 1-7
Location: Alpine A (Snowbird Center)
Date: Monday, August 2nd, 2004
Time: 9:30 AM
Abstract ID:568


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