Cytogenetic trait evolution and generic boundaries: evidence from Maxillariinae, a recently diversified Neotropical orchid clade

Chromosome numbers ranged from 2n = 36 to 2n = 76, with a predominance of counts between 2n = 36 and 2n = 42, consistent with x = 20 and recurrent dysploidy detected in the evolutionary analyses. Polyploidy was mainly detected at the population level and was often associated with niche shifts. Genome size appears to be relatively stable in Maxillariinae, with punctual increases and decreases across the phylogeny.

Although a marginal effect of heterochromatin block number on genome size variation was detected, PGLS models did not clearly support an association between genome size and any cytogenetic variable. No karyotype trait explained genome size variation; instead, Pagel’s λ value close to 1 suggests that genome size varies largely as a consequence of phylogenetic history, possibly reflecting changes in dispersed repetitive DNA sequences rather than changes in chromosome number or heterochromatin distribution alone.

Taken together, our cytogenetic analyses, literature survey, and time-calibrated phylogeny provide independent evidence relevant to generic boundaries in Maxillariinae, supporting the recognition of cytogenetically distinct lineages that may be obscured under broadly circumscribed generic concepts.