Plain Language Summary
Cycads are palm-like gymnosperms with a suite of distinctive
characteristics such as specialized relationships with insects, unique
toxicity, mobile sperm and specialized roots with nitrogen-fixing
microbes. We summarize the last decade of cycad research highlighting
insect mutualisms, functional microbial diversity in roots and leaves,
genomic evidence of horizontal transfer between microbes and cycads and
the evolutionary shift to non-motile sperm.
Summary
Cycads are an ancient lineage of gymnosperms that hold a special place
in seed plant evolution as sister, along with Ginkgo , to all
other gymnosperms. Cycads have myriad morphological, structural,
physiological, chemical and behavioral adaptations that position them as
a unique system to study genome evolution, symbioses, conservation and
microevolutionary processes. To this end, we have provided an overview
of cycad biology and recent advances in phylogenetics, symbiosis,
metabolomics and genomics to highlight their potential to address key
questions about the evolution of land plants.
苏铁这一古老类群在种子植物演化进程中具有独特地位,它与银杏一起构成其它裸子植物的姊妹类群。苏铁拥有大量形态学、结构、生理、化学和行为适应性,使它们成为研究基因组进化、共生关系、保护生物学和微观进化过程的独特系统。在此,我们提供了苏铁生物学的全面综述,对系统发育学、共生关系、代谢组学和基因组学的最新进展进行了总结,强调它们在解答陆地植物关键进化问题上的潜力。
Introduction
The great cycad biologist, Knut Norstog once suggested that we should
use “the analogy of the Rosetta Stone for the fund of information
stored within the living cycads and its importance to the interpretation
of plant biology….the very ancient structures and developmental
pathways of cycads enables us to make connections between the early
origins of seed plants and their present-day counterparts” (Donaldson,
2003). Indeed, cycads maintain a plethora of traits that have influenced
our understanding of land plant evolution, the origin of insect
pollination, symbiosis biology and coevolution (Fig. 1). Some examples
of these traits include: flagellated motile sperm, an ancestral land
plant trait; thermogenesis in reproductive tissues; symbiotic brood-site
pollination; specialized insect associates with diverse defensive
ecologies; and morphologically distinct coralloid roots for housing
nitrogen fixing microbiota (Fig. 1). For centuries, cycad research has
touched every corner of plant biology including ecology, evolution,
physiology, ethnobotany, phylogenetics, systematics, development,
genomics, signaling, and paleobotany. Due to recent methodological and
technical advances, cycad research is now, more than ever, uniquely
positioned to address basic questions across the biological sciences.
The rich paleobotanical and ethnobotanical cycad literature is beyond
the scope of this review, interested readers should see Carrascoet al. (2022) or Coiro et al. (2023) for recent examples.
Here we present the most recent advances in cycad research on land plant
evolution, the evolution and mechanisms of insect pollination and
herbivory, the biochemical basis of symbiosis, microbial symbionts, and
plant genomics.