Abstract
BackgroundHeterophyllous aquatic plants show marked phenotypic plasticity. They adapt to environmental changes by producing different leaf types: submerged, floating and terrestrial leaves. By contrast, homophyllous plants produce only submerged leaves and grow entirely underwater. Heterophylly and submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis, but little is known about the evolutionary outcome of habit. Recent evolutionary analyses suggested that rbcL, a chloroplast gene that encodes a catalytic subunit of RuBisCO, evolves under positive selection in most land plant lineages. To examine the adaptive evolutionary process linked to heterophylly or homophylly, we analyzed positive selection in the rbcL sequences of ecologically diverse aquatic plants, Japanese Potamogeton.Principal FindingsPhylogenetic and maximum likelihood analyses of codon substitution models indicated that Potamogeton rbcL has evolved under positive Darwinian selection. The positive selection has operated specifically in heterophyllous lineages but not in homophyllous ones in the branch-site models. This suggests that the selective pressure on this chloroplast gene was higher for heterophyllous lineages than for homophyllous lineages. The replacement of 12 amino acids occurred at structurally important sites in the quaternary structure of RbcL, two of which (residue 225 and 281) were identified as potentially under positive selection.Conclusions/SignificanceOur analysis did not show an exact relationship between the amino acid replacements and heterophylly or homophylly but revealed that lineage-specific positive selection acted on the Potamogeton rbcL. The contrasting ecological conditions between heterophyllous and homophyllous plants have imposed different selective pressures on the photosynthetic system. The increased amino acid replacement in RbcL may reflect the continuous fine-tuning of RuBisCO under varying ecological conditions.
Highlights
Many aquatic plants exhibit marked developmental plasticity, known as heterophylly
Conclusions/Significance: Our analysis did not show an exact relationship between the amino acid replacements and heterophylly or homophylly but revealed that lineage-specific positive selection acted on the Potamogeton rbcL
We demonstrated that the evolution of ecologically divergent Japanese Potamogeton has involved the molecular adaptation of rbcL
Summary
Many aquatic plants exhibit marked developmental plasticity, known as heterophylly. Heterophylly occurs widely across distantly related taxa and is thought to have arisen via convergent evolution [1]. Heterophyllous species have broad, thick leaves with stomata on the water surface (floating leaves), as well as elongated, thin leaves without stomata within the water column growing from submerged shoots (submerged leaves). They sometimes produce terrestrial leaves from land shoots during droughts. Heterophylly and submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis. Heterophyllous aquatic plants show marked phenotypic plasticity They adapt to environmental changes by producing different leaf types: submerged, floating and terrestrial leaves. And submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis, but little is known about the evolutionary outcome of habit. To examine the adaptive evolutionary process linked to heterophylly or homophylly, we analyzed positive selection in the rbcL sequences of ecologically diverse aquatic plants, Japanese Potamogeton
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