Evolution of the coniferous seed scale

Abstract

Background The Florin model is the commonly accepted theory of coniferous seed scale evolution. It describes the derivation of extant seed scale morphology from the morphology of fossil conifers via the reduction of complex to simple axillary structures. In this framework the seed scale is composed of a reduced lateral shoot with fertile and sterile appendages which are interpreted as leaf homologues. Scope The Florin model has three crucial problems that we (1) original derivation series does not take the ontogeny of extant conifers into account, (2) it cannot explain the morphology of all extant conifers, and (3) Taxaceae were originally excluded. Examination of seed cones of extant conifers shows that ovules occur in three different positions in the cone: in an axillary position, replacing a leaf, or terminating the cone axis. By interpreting the fertile appendage or seed-bearing structure as a leaf, not all positions are possible. The exclusion of Taxaceae from conifers is in stark contrast to molecular phylogenetic studies of recent decades, which include Taxaceae in conifers as sister to Cupressaceae. Therefore, the Florin model does not offer an adequate explanation for taxaceous morphology. Conclusion We conclude that the seed-bearing structure of conifers cannot be interpreted as homologous to a leaf. In the interpretation we present here the seed-bearing structure is the modified funiculus of the ovule, multiple of which laterally fuse to form the seed scale. The seed scales of all extant conifers can be derived from a Cunninghamia-like morphology via fusion and reduction of individual funiculi.