Independent genomic histories despite admixture from the same
gene pools
Although both M. spicata and M. × rotundifolia are
generally believed to stem from the same parental gene pools, our
admixture analyses suggest they have independent histories (Figure 2b).
While chromosome counts for M. spicata suggest that it is a
polyploid, reports of chromosome counts for M. ×
rotundifolia are rare but do point to a diploid status (Chambers &
Hummer, 1994; Harley & Brighton, 1977). The usage of herbarium material
prevented cytological evaluations of our samples and we therefore
bioinformatically estimated ploidy using our sequencing data. Our
estimates are not conclusive but in combination with our admixture
analyses they do support that M. spicata is an allopolyploid and
that M. longifolia is likely cytomictic with both diploid and
polyploid populations (Figure 2 and Table 1). However, the ploidy level
of M. x rotundifolia is more uncertain as most were
assigned as “likely polyploid” (triploids; Table 1). The existence of
triploid individuals among the analyzed specimens is not unlikely.
However, it is unclear to what extent HMMploidy is able to distinguish
between triploids and tetraploids. For example, tetraploids formed from
the merging of the very closely related genomes of M. longifoliaand M. suaveolens could potentially appear as triploids due to
extensive allele sharing between them (Figure 3). But it is unclear why
this would more often be the case for specimens morphologically assigned
as M. x rotundifolia (Table 1). It is possible that the
distinct genomic histories of M. spicata and M. xrotundifolia are linked to differences in ploidy but no firm
conclusions can be made based on our data.
Differences in frequencies of sexual reproduction can also drive the
genomic distinction between M. × rotundifolia and M.
spicata . While M. × rotundifolia exhibits a reduction in
fertility, M. spicata is usually completely fertile (Table S3;
Harley & Brighton, 1977). It is therefore possible that M. ×rotundifolia to a higher degree propagates clonally via rhizomes.
However, such a scenario is not completely supported by our plastome
analysis that suggests that most of the analyzed M. ×rotundifolia stem from independent hybridizations while the
plastomes of M. spicata are largely homogeneous suggesting a
single origin (Figure S3). It is however likely that human imposed
selection on M. spicata in cultivation coupled with clonal
propagation of plants with desired traits (Harley & Brighton, 1977)
have caused shifts in allele frequencies in M. spicata and that
the distinct genomic histories of these taxa, in addition to potentially
be associated with ploidy differences, also are coupled to the (partly)
cultivation status of M. spicata .