Polyglutamine variation in a flowering time protein correlates with island age in a Hawaiian plant radiation

Abstract

Background: A controversial topic in evolutionary developmental biology is whether morphological diversification in natural populations can be driven by expansions and contractions of amino acid repeats in proteins. To promote adaptation, selection on protein length variation must overcome deleterious effects of multiple correlated traits (pleiotropy). Thus far, systems that demonstrate this capacity include only ancient or artificial morphological diversifications. The Hawaiian Islands, with their linear geological sequence, present a unique environment to study recent, natural radiations. We have focused our research on the Hawaiian endemic mints (Lamiaceae), a large and diverse lineage with paradoxically low genetic variation, in order to test whether a direct relationship between coding-sequence repeat diversity and morphological change can be observed in an actively evolving system.

Results: Here we show that in the Hawaiian mints, extensive polyglutamine (CAG codon repeat) polymorphism within a homolog of the pleiotropic flowering time protein and abscisic acid receptor FCA tracks the natural environmental cline of the island chain, consequent with island age, across a period of 5 million years. CAG expansions, perhaps following their natural tendency to elongate, are more frequent in colonists of recently-formed, nutrient-rich islands than in their forebears on older, nutrient-poor islands. Values for several quantitative morphological variables related to reproductive investment, known from Arabidopsis fca mutant studies, weakly though positively correlate with increasing glutamine tract length. Together with protein modeling of FCA, which indicates that longer polyglutamine tracts could induce suboptimally mobile functional domains, we suggest that CAG expansions may form slightly deleterious alleles (with respect to protein function) that become fixed in founder populations.

Conclusion: In the Hawaiian mint FCA system, we infer that contraction of slightly deleterious CAG repeats occurred because of competition for resources along the natural environmental cline of the island chain. The observed geographical structure of FCA variation and its correlation with morphologies expected from Arabidopsis mutant studies may indicate that developmental pleiotropy played a role in the diversification of the mints. This discovery is important in that it concurs with other suggestions that repetitive amino acid motifs might provide a mechanism for driving morphological evolution, and that variation at such motifs might permit rapid tuning to environmental change.

Figure 1

FCA homolog sequence alignment around the WW domain. The WW domain is shown boxed, with probable secondary structure marked (arrows for beta strands). The hypothesized beta-helix between the second RNA recognition motif (RRM) and the WW domain, and extending past the WW, is marked with a thick green line under the alignment. The polyglutamine region explored here is shown boxed, and with asterisks above amino acid residues. Sequences shown in the alignment: T. aestivum (Triticum), AAP84419 and AAP84418; L. perenne (Lolium), AAT72460; O. sativa (Oryza), AAW62371; H. vulgare (Hordeum), AAF97846; S. officinarum (Saccharum), CA085029; A. thaliana (Arabidopsis), AAW38964; B. napus (Brassica), AAL61622; P. sativum (Pisum), AAX20016; M. truncatula (Medicago), ABE82791; Z. elegans (Zinnia), AU291241; and S. rugosa (Stenogyne), EU005232. See Additional file 1 for the complete protein alignment and further explanation of structural features.

Figure 2

A partial FCA protein sequence alignment of selected mint taxa. The polyQ stretch (orange) is directly C-terminal of the WW domain. Stenogyne cranwelliae 1 and Phyllostegia hispida are homozygotes, confirming the base pair/Q-tract calibration.

Figure 3

Island-island frequency distributions of FCA-like alleles among the Hawaiian mints. Note the right → left shift in frequency distributions as islands age, from Hawai'i to Kaua'i. All alleles from each individual were pooled by island. bp, base pairs.

Figure 4

Average allele lengths of FCA homologs shift with island age in the Hawaiian chain. Longer alleles are more frequent on younger islands. The mean differences are statistically significant (Kruskal-Wallis and ANOVA, P < 0.001). See Table 1 for island-island Tamhane's tests. X-axis, island age (in millions of years, decreasing, as indicated next to representative volcanoes [12]). Whiskers indicate ± 0.5 standard deviations around the means.

Figure 5

Frequency distributions of SSR alleles for two additional loci. Unlike the Hawaiian mints' FCA-like locus, frequency distributions of SSR alleles for two additional loci do not show archipelago-wide geographic progression. A, unigene 260708 (no annotation); B, unigene 261064 (annotated as At4g23400.1-major intrinsic family protein/MIP family protein [16]). Insets, average allele lengths for each island with ± 0.5 standard deviations. As described previously [16], the frequency distribution for A shows both left and right tails, representing samples principally from the island of Hawai'i. In B, the allele frequency distribution is substantially right-shifted, the four longest alleles representing a single taxon from Maui Nui (Stenogyne bifida). Numbers of individuals genotyped for A and B, respectively, were 93 and 91. A, Kruskal-Wallis and ANOVA n.s.; B, Kruskal-Wallis P < 0.05, ANOVA P < 0.001. B, Tamhane's T2 is significant only for the Maui Nui/Hawai'i post hoc comparison, P < 0.05.

Figure 6

Linear regressions of five reproductive morphological traits against FCA homolog average allele lengths. Five reproductive morphological traits show similar linear correlations with FCA homolog average allele length per Hawaiian mint individual. The x-axis is average allele length, and the y axis represents measurements in millimeters. Dark blue = nutlet size; green = corolla lower lip length, light blue = corolla upper lip length, red = number of flowers per verticillaster, yellow = pedicel length. Regression lines, from top to bottom at the y-intercept,: pedicel length, flowers per verticillaster, corolla upper lip length, corolla lower lip length, nutlet size. See Table 2 for R² and significance values. Note that none of these morphological variables show significant correlation with average allele lengths for the other two loci shown in Figure 5.