Unearthing modes of climatic adaptation in underground storage organs across Liliales

Testing adaptive hypotheses about how continuous traits evolve in
association with developmentally-structured discrete traits, while
accounting for the confounding influence of other, hidden, evolutionary
forces, remains a challenge in evolutionary biology. In one example of
this, geophytes are herbaceous plants capable of retreating underground
and use underground storage organs (USOs) to survive extended periods of
unfavorable conditions. Such plants have evolved multiple times
independently across all major vascular plant lineages. Even within
closely related lineages, however, geophytes show impressive variation in
the morphological modifications and structures (i.e., “types” of USOs)
that allow them to survive underground. Despite the developmental and
structural complexity of USOs, the prevailing hypothesis is that they
represent convergent evolutionary “solutions” to a common ecological
problem, though some recent research has drawn this into question. We
extend existing phylogenetic comparative methods to test for links between
geophytes’ hierarchical discrete morphological traits associated with USOs
and adaptation to environmental variables, using a phylogeny of 621
species in Liliales. We found that plants with different USO type do not
differ in climatic niche more than expected by chance, with the exception
of root morphology, where modified roots are associated with lower
temperature seasonality. These findings suggest that root tubers may
reflect adaptations to different climatic conditions than those
represented by other types of USOs. Thus, the tissue type and
developmental origin of the USO structure may influence the way it
mediates ecological relationships, which draws into question the
appropriateness of ascribing broad ecological patterns uniformly across
geophytic taxa. This work provides a new framework for testing adaptive
hypotheses and for linking ecological patterns across morphologically
varying taxa while accounting for developmental (non-independent)
relationships in morphological data.