Segmental Duplication

Medicago, like many other legumes, has a whole genome duplication event (aka polyploidy) within its lineage. This particular WGD event is dated to have occurred approximately 58 million years ago and contributed significantly to the genome we see today. The increase of duplicate genes and subsequent functional divergence might have supported the evolution of nodulation and symbiotic nitrogen fixation.

Extensive gene fractionation between WGD-derived homeologues has left few remnants of the event. The figure below illustrates the small synteny blocks that are typical between medicago chromosomal regions.
Significant macrosynteny has been described among legume species. Conserved blocks, sometimes as large as chromosome arms, span major portions of the euchromatin. A given Medicago region is typically syntenic with one other Medicago region (though usually in small blocks showing degraded synteny), four Glycine max regions, one or two Lotus japonica regions, and three Vitis vinifera regions (dot plots not shown).