Parasitic insect herbivores
Cycads’ unique chemistries are also important for understanding cycads’
associations with parasitic insects. Cycads produce potent
lineage-specific phytotoxins such as methylazoxymethanol (MAM)
glycosides, β-methylamino-L-alanine (BMAA), and lesser studied compounds
such as steryl glycosides. Yet cycad-feeding has been documented in at
least six insect orders, with most cycad herbivores belonging to the
orders Lepidoptera (moths and butterflies), Coleoptera (beetles and
weevils), and Hemiptera (true bugs). A previous review concluded that
cycad-feeding has evolved independently among Lepidoptera multiple
times, and that insect defensive traits may be especially important in
determining whether cycad-feeding lineages diversify (Whitaker &
Salzman, 2020). A systematic review of cycad-feeding Coleoptera is
currently underway to investigate the ecology and evolution of their
incredible diversity of feeding ecologies, species interactions, and
defensive traits.
Many cycad-associated insects are obligate cycad specialists and must
possess adaptations to contend with cycads’ chemical defenses. In
general, herbivorous insects can excrete, sequester, or detoxify
phytotoxins, but it is not clear which strategies most cycad herbivores
utilize. There is some evidence that pollinating weevils excrete BMAA in
their frass and pupal casings, and some Lepidoptera have been shown to
sequester MAM for protection from natural enemies (reviewed in Whitaker
& Salzman, 2020). Some cycad-feeding Aulacoscelinae beetles reflexively
bleed MAM-glycosides – presumably sequestered from their foodplants –
when disturbed or threatened (Prado et al., 2011), and at least
one lepidopteran has been shown to be capable of detoxifying MAM (Teas,
1967). Recently, the obligate cycad herbivore Eumaeus atalabutterfly was found to accumulate BMAA into their larval and adult
tissues (Whitaker et al ., 2023), although the defensive value of
sequestering BMAA remains questionable given its latent toxicity
(Whitaker et al., 2022). Genomic evidence does suggest, however,
that toxin tolerance is a key adaptation in the radiation ofEumaeus butterflies, a wholly cycadivorous neotropical genus of
six species (Robbins et al., 2021). It has also been suggested
that insects’ gut microbiomes may contribute to degrading cycad toxins
(Salzman et al ., 2018), though this remains to be experimentally
demonstrated.
Specialized cycad herbivores must also possess adaptations to locate and
select feeding and oviposition sites, but very little is known about the
chemical, thermal, and visual cues used in host selection. A more
mechanistic view of host selection would help assess the potential for
host switching and pest management as increasing cultivation of
non-native cycads introduces opportunities for emerging pest dynamics
(Rensburg et al ., 2023). Invasive pests such as the cycad scale
represent major threats to cycad conservation (Marler, Lindström &
Watson, 2021), and even native herbivores may pose a threat in some
circumstances: recent host expansions have been documented among
cycad-feeding lepidoptera where native and non-native cycads are
co-cultivated, with potentially dire effects for cycad conservation
(Marler, Lindström & Terry, 2012; Normark et al., 2017; Whitakeret al ., 2020). Cycad-herbivore interactions span from mutualistic
to parasitic and provide the opportunity for investigating mechanisms of
host selection, specialization, and toxin tolerance, a better
understanding of which will improve species management and advance
research on coevolution and plant-insect interactions.
Novel insights into the evolution of coralloid roots
symbiosis
One of the first observations of cycad root symbionts is from the 19th
century, in which various taxa were described by Schneider (1894),
including fungi, bacteria and algae. Subsequent research however,
focused mostly on Nostocales cyanobacteria as the key taxon in
coralloid roots (Suárez-Moo et al ., 2019; Bell-Doyon et
al ., 2020). These cyanobionts colonize a morphologically distinct cycad
organ called the coralloid root (Fig. 1h). CyanobacterialNostocales in the coralloid root fix nitrogen in exchange for
plant carbohydrates, but only a few species from the generaNostoc, Desmonostoc, Calothrix and Aulosira have been
found inside the coralloid root forming three distinct phylogenetic
clades (Bustos-Diaz et al ., in review; Fig. 3). While no
“symbiotic genes” have been identified in cyanobiont genomes,
experiments using cyanobionts isolated from cycads to infect other
plants found multiple genes crucial for the establishment of the
symbiosis (Alvarez et al. , 2022; Wong & Meeks, 2002), suggesting
that while specific traits are required for the symbiosis, gene
specificity, if any, is not apparent with current data (Bustos-Diazet al. , in review).
The mechanism of initiating symbiosis and the necessary morphological
changes in both plant and symbiont is not well described, although
recent evidence shows that multiple molecular mechanisms and signal
recognition systems in both the cyanobiont and the host underlie the
symbiosis, including the hormogonium–inducing factor produced by
precoralloid roots (Hashidoko et al. , 2019; Figs. 1g & 3a).
Based on Cycas panzhihuaensis coralloid roots transcriptomes (Liuet al ., 2022), additionally, terpenoids have been implicated with
establishment of the symbiosis. The bacterial community has also been
shown to produce various metabolites which might influence the symbiosis
(Fig. 3a; Dehm et al. , 2019; Freitas et al ., 2022).
Amongst these metabolites, BMAA has been linked to impaired
cyanobacterial nitrogen fixation (Berntzon et al ., 2013) and may
be produced by both the cyanobiont and host (Marler, Snyder & Shaw,
2010). The main challenge here is analytically distinguishing amongst
the vast number of BMAA isomers and deciphering their interactions with
other toxic cycad metabolites.
We are beginning to explore the role of gene expression in symbiosis .
The recently sequenced cycad genome (see below) recovered a two-fold
upregulation of 10 genes from the common symbiotic pathway of
legume-rhizobial and plant-mycorrhizal symbiosis (Delaux & Schornack,
2021) (RAD1 , DHY , SymRK , EPP1 ,
VAPYRIN , CASTOR/POLLUX , NFP , CYTB561 ,GRAS , HEP ) in precoralloid roots compared to mature
coralloid roots colonized by cyanobacteria. In contrast, five genes were
highly expressed in colonized coralloid roots (CCaMK ,CYCLOPS , LIN , SYN and TAU ), while a large
number of genes are preferentially up and down regulated in both root
types (Fig. 3b). In addition, 24 conserved genes were found shared among
plants with cyanobacterial symbiosis (Table 1). The specific role of all
of these plant genes for symbiosis remains to be experimentally
elucidated. Comparative genomics provides the necessary groundwork for
an understanding of the evolutionary mechanisms of plant-cyanobacterial
symbiosis.