Alloteropsis (from the Greek allotrios ("strange") and opsis ("appearance")) is a genus of Old World plants in the grass family.
The group is widely distributed in tropical and subtropical parts of Africa, Asia and Australia, as well as on certain islands in the Indian and Pacific Oceans. The genus is unusual among plants in that it includes species with both C<sub>3</sub> and C<sub>4</sub> photosynthetic pathways, and ongoing research is investigating these taxa as a case study in how carbon concentrating mechanisms for photosynthesis evolve in land plants.
Most of the species of Alloteropsis use variants of the C<sub>4</sub> photosynthetic pathway, but A. semialata ssp. eckloniana uses the C<sub>3</sub> photosynthetic pathway. Phylogenetic reconstructions of the evolutionary relationships between these species have led to two hypotheses about how photosynthetic pathways have evolved within the group. First, C<sub>4</sub> photosynthesis evolved in three lineages within this group, leading to independently derived realisations of this pathway (the hypothesis of multiple C<sub>4</sub> origins). Secondly, that there was a single origin of C<sub>4</sub> photosynthesis within the genus, and the C<sub>3</sub> taxon, A. s. ecklonia, was subsequently derived from a C<sub>4</sub> ancestor (the reversion hypothesis). Since C<sub>4</sub> photosynthesis is a complex trait, its evolution followed by a reversion to the ancestral type of C<sub>3</sub> photosynthesis would represent an exception to Dollo's law.
The reversion hypothesis is the most parsimonious explanation of phylogenetic relationships within Alloteropsis. However, direct evidence for the hypothesis, in the form of C<sub>4</sub> genes or pseudogenes in the C<sub>3</sub> taxon, is currently lacking. Instead, two pieces of evidence better support the hypothesis of multiple C<sub>4</sub> origins. First, different variants of C<sub>4</sub> leaf anatomy are found in three different Alloteropsis lineages. Secondly, key C<sub>4</sub> enzymes (PEPC and PEPCK) were recruited multiple times to function in C<sub>4</sub> biochemistry across independent lineages.
C<sub>4</sub> photosynthetic pathway evolution in Alloteropsis also represents an example of adaptive evolution via horizontal gene transfer in eukaryotes. It is the first such example of gene transfer between plant species that are not in direct physical contact (as in a host-parasite relationship). In each case, genes adapted for an important function in C<sub>4</sub> photosynthesis have been transferred from grass lineages that diverged from Alloteropsis more than 20 million years ago, and independently evolved C<sub>4</sub> photosynthesis. Horizontally inherited genes encode the photosynthesis enzymes PEPC and PEPCK. All other genes expressed in the mature C<sub>4</sub> leaf of A. s. semialata were vertically inherited from a common ancestor with the C<sub>3</sub> taxon A. s. eckloniana.