Insights into species delimitation of selected species in the flowering plant genus Medicago section Buceras (Leguminosae)

The genus Medicago (Leguminosae, Papilionoideae) contains about 90 species
including the important forage crop alfalfa Medicago sativa and the
genomic model Medicago truncatula. Despite intensive research on the genus
because of its agricultural importance, there is a relative lack of
information about chromosome number and genome size in some Medicago
species, especially those from section Buceras that were formerly placed
in the sister genus Trigonella, and are paraphyletic to the remainder of
the genus Medicago. Past studies revealed that previous species
delimitations did not conform well with complex patterns of morphological
or genetic variation. Some published chromosome numbers, e.g., 2n = 28 and
2n = 44, differ from those of the rest of the genus, which are mostly 2n =
16 or polyploids thereof, although some cases of aneuploid reduction or
dysploidy (e.g., 2n = 14) do exist. Here we estimated phylogenetic
relationships of 42 accessions corresponding to 14 currently recognized
Medicago species that are paraphyletic to the remainder of Medicago with a
focus on Medicago monantha; for a number of those accessions, we obtained
estimates of genome size (39) and chromosome number (14). We can confirm
the delimitation of two species within section Buceras, and our data
suggest that there are at least two entities with distinct geographic
distributions within the currently recognized species M. monantha, which
differ in chromosome number and genome size. Our data also suggest that
polyploidy and post-polyploid descending dysploidy played a significant
role in genome evolution within section Buceras. Our data provide a strong
foundation for whole-genome sequencing projects and further in-depth
research of these paraphyletic lineages.