Not all ‘pine cones’ flex: functional trade‐offs and the evolution of seed release mechanisms

Abstract

Seed dispersal is critical for plants, but the evolution of mechanisms that actually release seeds from their parents is not well understood. We use the reproductive cones of conifers, specifically the Pinaceae clade, to explore the factors driving the evolution of different release mechanisms in plants.We combine comparative anatomical and phylogenetic analyses to test whether fundamental trade-offs in the mechanical and hydraulic properties of vasculature underlie the evolution of two seed release mechanisms: cone scale flexion and cone scale shedding. We then test whether these mechanisms are linked with differences in seed size, dispersal syndrome and reproductive allocation.Cone scale xylem in flexing species is tough, but poorly conductive. Xylem in shedding species is less extensive, fragile and highly conductive; its thin-walled tracheids allow scales to easily fracture at maturity. Shedding is also consistently associated with large, densely packed seeds.Pinaceae cones exploit a well-known trade-off in xylem mechanical strength vs hydraulic efficiency to generate release mechanisms that allow seeds of various sizes to leave the protecting cone. The linkage among release mechanisms, vascular anatomy and seed traits illustrates how a wide variety of selective pressures may influence the function and physiology of reproductive structures.

Early Cretaceous abietoid Pinaceae from Mongolia and the history of seed scale shedding

“…A large number of Pityostrobus species have been described based on fossil cones from Early Cretaceous localities, but none shows any indication of a shedder dispersal (e.g., Herrera et al, 2016; Smith et al, 2016; Gernandt et al, 2018; Losada et al, 2018). Most have also been described on the basis of permineralized material, which can be challenging to compare with the abundant lignified specimens on which L. mellonae is based.…”

“…A feature that has been overlooked in studies of Pinaceae but that may be relevant to understanding the early evolutionary history of the group is the mode of seed dispersal. Losada et al (2018) found a set of anatomical features that help to explain the mechanism for the disarticulation of the seed cones in several genera of Pinaceae. In cones of “shedders,” including those of the genera Abies, Cedrus, Keteleeria , and Pseudolarix , there is only a small amount of xylem, composed of thin‐walled tracheids with wide lumens and high hydraulic efficiency, near the base of the cone scales.…”

“…In cones of “shedders,” including those of the genera Abies, Cedrus, Keteleeria , and Pseudolarix , there is only a small amount of xylem, composed of thin‐walled tracheids with wide lumens and high hydraulic efficiency, near the base of the cone scales. Bract–scale complexes are easily shed from the cone axis and release the winged seeds at maturity (Losada et al, 2018). In addition, Abies and Cedrus appear to have a consistent area at the base of the scale where abscission occurs, while Keteleeria and Pseudolarix do not (J. M. Losada, Instituto de Hortofruticultura Subtropical y Mediterranea, personal observation).…”

“…In addition, Abies and Cedrus appear to have a consistent area at the base of the scale where abscission occurs, while Keteleeria and Pseudolarix do not (J. M. Losada, Instituto de Hortofruticultura Subtropical y Mediterranea, personal observation). In “flexers,” which include all pinoids plus Tsuga and Nothotsuga , the proximal portion of the cone scale xylem is well developed, tough, and relatively poorly conductive (Losada et al, 2018). Shedding seed cones occur in about a quarter of living Pinaceae species, and the phylogenetic distribution of this feature suggests that it may have been important during the early diversification and expansion of the group.…”

Reconstructing Krassilovia mongolica supports recognition of a new and unusual group of Mesozoic conifers

“…As in many extant broadleaved conifers the leaves probably abscised and fell at the end of their useful life [84]. In contrast, the lack of prominent xylem near the base of Krassilovia bract-scale complexes, and the relatively large lumens and thin walls of the xylems cells, suggest a more precise mode of dehiscence ( Figs 6 and 7), analogous, for example, to the disarticulation of the cones and shedding of bract-scale complexes that occurs extant Abies and Cedrus [85]. A well-organized mode of shedding is also consistent with the very regular, distinct scars seen on dispersed Krassilovia seed cone axes (Fig 1D).…”