Abstract Detail

Symbioses: Plant, Animal, and Microbe Interactions

Doyle, Jeff J. [1], Powell, Adrian F. [2].

The origins of legume nodulation symbioses: a conundrum involving homology, exaptation, and polyploidy.

How novel structures and processes originate is a key question in evolutionary developmental genetics. It is now understood that in many if not most cases, co-option (exaptation) of components from pre-existing developmental genetic programs is involved; parallel recruitments can lead to so-called “deep homologies”. Conversely, deep homologies due to parallel exaptation complicate assessment of overall homology in phylogenetically distant taxa. Symbiotic nitrogen fixation occurs sporadically throughout the so-called “Nitrogen-fixing clade” of rosids, and is prevalent in legumes (Leguminosae). Plant macrosymbionts produce novel organs (nodules), typically on roots, that house taxonomically diverse symbiotic nitrogen-fixing bacterial microsymbionts (“rhizobia”). Phylogenetic optimizations of nodulation, as well as anatomical and morphological evidence, suggest multiple independent origins of nodulation in various rosid groups, but deep homologies exist at the molecular level, notably in the early signalling stages that have been recruited from the older and more phylogenetically widespread mycorrhizal symbiosis. In legumes, nodulation is clearly apomorphic, occurring in the two sister clades (Papilionoideae and the Mimosoideae-Caesalpinieae-Cassiinae [MCC] clade) that comprise the bulk of the family. Nodulation is characteristic of all but the earliest-diverging lineages of papilionoids, and is similarly widely distributed in mimosoids, but is sporadic elsewhere in the MCC clade. This raises the question of the number of independent origins of nodulation in the family, and has led to a search for criteria to test the homologies of nodules in phylogenetically separated taxa (e.g., mimosoids and papilionoids). But, in addition to technical concerns with applying various molecular and morphological criteria, the deep homologies expected to occur from parallel exaptation complicate homology assessment even more seriously given shorter time scales and more phylogenetically proximal distribution of nodulation in legumes. An alternative approach would be to distinguish between secondary loss of nodulation and primary absence in non-nodulating taxa. This involves searching for “fossil” evidence in the genomes of non-nodulating taxa. In papilionoids it has been suggested that polyploidy played a role either in the origin or the refinement of nodulation. Recent results suggest that separate polyploidy events occurred in the papilionoids, the MCC clade, and early in the evolution of some strictly non-nodulating lineages. Patterns of homoeologous gene retention and recruitment might also provide evidence of nodule homologies.

1 - Cornell University, 412 Mann Library Building, Ithaca, NY, 14853-4301, USA
2 - Cornell University, Plant Biology, 412 Mann Library, Ithaca, NY, 14850, USA

Keywords:
nodulation
homology
Leguminosae
Polyploidy.

Presentation Type: Oral Paper:Papers for Topics
Session: 36
Location: Salon 5/The Shaw Conference Centre
Date: Tuesday, July 28th, 2015
Time: 8:30 AM
Number: 36003
Abstract ID:444
Candidate for Awards:None