Multiple innovations underpinned branching form diversification in mosses
Résumé
Broad-scale evolutionary comparisons have shown that branching forms arose by convergence
in vascular plants and bryophytes, but the trajectory of branching form diversification
in bryophytes is unclear. Mosses are the most species-rich bryophyte lineage and
two sub-groups are circumscribed by alternative reproductive organ placements. In one,
reproductive organs form apically, terminating growth of the primary shoot (gametophore)
axis. In the other, reproductive organs develop on very short lateral branches. A
switch from apical to lateral reproductive organ development is proposed to have primed
branching form diversification.
Moss gametophores have modular development and each module develops from a single
apical cell. Here we define the architectures of 175 mosses by the number of module classes,
branching patterns and the pattern in which similar modules repeat. Using ancestral character
state reconstruction we identify two stages of architectural diversification.
During a first stage there were sequential changes in the module repetition pattern, reproductive
organ position, branching pattern and the number of module classes. During a second
stage, vegetative changes occurred independently of reproductive fate.
The results pinpoint the nature of developmental change priming branching form diversification
in mosses and provide a framework for mechanistic studies of architectural diversification