Natural Interspecific Hybridization Between Mimosa benthamii and Mimosa mollis (Fabaceae): Evidence for the Recognition of Mimosa × gretheriae nothosp. nov.

Natural hybridization can obscure species boundaries and generate morphologically atypical individuals whose taxonomic identity cannot be resolved from phenotype alone. In central Mexico, Mimosa benthamii and M. mollis occur in sympatry with individuals exhibiting unusual combinations of morphological traits. We integrated morphological and molecular evidence to test whether these individuals originated through interspecific hybridization, characterize variation in parental genetic contributions, and evaluate whether they constitute a morphologically diagnosable hybrid entity. We examined 29 leaf, prickle, fruit, and inflorescence characters in 145 plants and analyzed 42 reproducible RAPD bands, including 16 parental-diagnostic markers. The putative parental species were morphologically differentiated and distinguishable by parental-diagnostic RAPD markers, whereas morphologically atypical individuals exhibited heterogeneous combinations of intermediate, parental-like, and transgressive morphological characters and occupied intermediate and partially overlapping positions relative to both taxa. The concordance between morphological and molecular evidence supports natural hybridization between M. benthamii and M. mollis. Hybrid-index analyses revealed heterogeneous parental-marker profiles, including individuals with intermediate values and others showing greater similarity to M. mollis. However, the limited resolution of RAPD markers precluded confirmation of backcrossing events, genetic introgression, or specific hybrid generations. Despite the occurrence of mixed parental-marker profiles, M. benthamii and M. mollis remained morphologically differentiated and distinguishable by parental-diagnostic RAPD markers within the contact zone. The concordance between molecular evidence consistent with hybrid ancestry and a diagnostic combination of morphological characters supports recognition of the hybrid entity as Mimosa × gretheriae nothosp. nov. These findings document natural interspecific hybridization in Mimosa and show that hybridization can generate substantial phenotypic and molecular variation while the parental taxa remain morphologically distinguishable.