Developmental timing in the female reproductive cycle of Araucaria araucana: seasonality and evolutionary perspectives

Abstract

• Background and Aims Shifts in the timing and rate of reproductive development have played a central role in plant evolution, often being shaped by seasonal climates. Araucariaceae provides a valuable opportunity to study developmental timing across climates: while most species inhabit warm regions, Araucaria araucana occurs in a temperate region with marked seasonality. We anatomically describe its previously undocumented female reproductive cycle, considering seasonality as a selective pressure on ontogeny. • Methods We sampled female cones of Ar. araucana periodically during their development. We resinembedded, sectioned, stained (toluidine blue, PAS) and photographed the developing ovules. We compared these results with published studies on Araucaria angustifolia and Agathis australis. • Key Results Female cone development in Ar. araucana follows a sequence of clearly distinguishable anatomical stages, which are consistent with those described for other conifers. These include seed scale and nucellus formation, megaspore mother cell differentiation, gametophyte development, archegonia maturation, fertilization, and embryo formation. Ovule development followed an evolutionarily conserved sequence across Araucariaceae; however, Ar. araucana showed distinct timing and rate: its cycle was shorter, ovule development started later but progressed faster, and pollination occurred at a later ovule stage, followed by accelerated pollen tube growth. These shifts likely re•ect adaptation to temperate climates through the alignment of development with favourable climatic conditions. A comparable shift was observed in cone growth: while Ar. angustifolia delayed growth until after fertilization, Ar. araucana started early but paused in winter, likely an efScient resource strategy in its seasonal environment. • Conclusions Our study provides the Srst comparative analysis of female cone development timing in Araucariaceae. While ovule development appears structurally conserved across the family, its timing and resource allocation strategies differ among species, likely re•ecting climatic adaptations. More broadly, these Sndings may re•ect the evolutionary potential of the conifers’ female reproductive cycle to respond to environmental pressures, which is particularly relevant in the context of climate change.