INTRODUCTION
Coevolution —the reciprocal adaptation between two or more interacting
organisms (Hembry et al. 2014)— is considered to have played a central
role in shaping the diversity of life on Earth (Laine 2009). Yet,
empirical evidence that coevolution is the main mechanism driving
reciprocal diversification among interacting clades lack alternative
explanations (Medina et al. 2022). Most examples of diversification
driven by coevolution involves cases of resource competition between
species during adaptive radiations (reviewed in Hembry et al. 2014). In
contrast, examples of diversification arising from mutualistic and
antagonistic interactions at different trophic levels are comparatively
rare (e.g. Hutchinson et al. 2017; Fuzessy et al. 2022). The lack of
attention to such pervasive and relevant types of interactions not only
leads to a biased view of the importance of coevolution in
diversification processes, but also limits our understanding on how
complex relationships between species arise within communities (Thompson
1994).
A major obstacle to generate empirical evidence for coevolutionary
history in antagonistic/mutualistic interactions has been ruling out
alternative explanations of diversification (Althoff et al. 2014; Medina
et al. 2022). Although often taken as evidence for coevolution,
reciprocal adaptation can emerge through adaptation by one of the
interacting species alone or through neutral processes, such as genetic
drift (Yoder & Nuismer 2010). Plant-animal interactions are recognized
as important drivers of one-sided evolution (Strauss & Irwin 2004;
Hembry & Weber 2020; Johnson et al. 2021). The correlation in features
of interacting pairs, such as floral structures that match traits of
pollinators (Nilsson 1988), color vision phenotypes that shape fruit
color (Valenta et al. 2018), or defenses against predators that are met
by counter defenses (Brodie & Ridenhour 2003), may also arise through
correlated selection (Nuismer et al. 2010). Phylogenetic congruence
between the interacting lineages, another major expectation of
coevolutionary processes, is neither exempt of problems. While often
interpreted as evidence of co-speciation, in nature, other types of
evolutionary events often act concurrently, so a perfect cophylogenetic
fit is rarely observed (Hutchinson et al. 2017; Fuzessy et al. 2022).
Congruence may result from a combination of events unrelated to
co-speciation, such as phylogenetic tracking (when one taxon tracks the
evolution of its interaction partner), shared biogeographic histories
(like vicariance, where common biogeographic events cause the
subdivision of the distribution range of both interacting partners) or
from parallel evolution driven by climate changes and other broad
selective agents (Nuismer et al. 2010).
Demonstrating that interactions shape concomitant diversification on
both sides has also been challenging due to two main constraints. First,
interacting species on both sides are on a continuum of
generalist-specialist interactions, which strongly affects the expected
selective pressure each party exerts on the other, and hence the overall
selective regime. In plant-animal interactions, phylogenetic tracking by
adaptive switching behaviors, the ability of generalists to adaptively
increase the consumption of one resource at the expense of decreased
consumption of an alternative resource (Carnicer et al. 2008), is
expected to be particularly relevant in creating phylogenetic
congruences that can be confounded as coevolutionary diversification.
This is because switching behaviors should not only facilitate animals
to change feeding habits to new plants at the expense of others, but
morphology and other constraints should also increase the chances that
changes will favor plants that are closely related (and thus more
similar) to those previously used as a resource (see Carnicer et al.
2008). The second constrain is that mutualism and antagonism themselves
are only two ends of a single continuum, where no individual acts
exclusively as an antagonist or a mutualist. For example, animals that
feed on flowers can carry a significant amount of pollen in their body,
acting simultaneously as florivores and pollinators (Kress et al. 1994).
In the same vein, when feeding from fruits, many frugivores transport
the seeds far from parent trees, acting as seed dispersers (Schupp et
al. 2010). However, since not all seeds are defecated intact, seed
dispersing animals can also act as seed predators (Norconk 2020). Due to
these constrains and the difficulties to rule out alternative
explanations, the role of coevolutionary processes in
antagonistic/mutualistic interactions is currently backed by limited
evidence (but see Hutchinson et al. 2017; Fuzessy et al. 2022).
While generating macroevolutionary evidence for coevolutionary
diversification in antagonistic/mutualistic interactions remains
challenging, recent advances in analytical tools developed for testing
phylogenetic congruence provides a stronger framework to rule out
alternative explanations of diversification (Balbuena et al. 2013;
Blasco-Costa et al. 2021). A great advantage of these cophylogenetic
methods (so called pattern-based methods, Dismukes et al. 2022) is the
ability to use phylogenies where interacting species may have multiple
pairs, which is the case of most plant-animal interactions. This allows
to accommodate variation among the interacting species along the
generalist-specialist continuum while considering phylogenetic
uncertainties. With this framework, it is also possible to evaluate the
contribution of individual links between interacting partners to the
overall congruence (Hutchinson et al. 2017), and to test the extent to
which the patterns of co-diversification and/or coevolution can be
explained by phenotypic traits (Blasco-Costa et al. 2021).
Here, we adopt the above cophylogenetic framework to investigate
macro-coevolutionary hypotheses for phylogenetic congruence of
plant-lemur interactions in Madagascar. The flora and fauna of
Madagascar provides an interesting study case to test such
macro-coevolutionary hypotheses. About 82% of vascular plants
(Callmander et al. 2011) and 84% of vertebrates (Goodman & Benstead
2005) are endemic, and hence have evolved under similar climatic
conditions within a relatively small island, and have had ample
opportunity for interaction and coevolution (Génin et al. 2022). Lemurs,
in particular, diverged from Haplorrihini primates about 68 Ma, having
originated from an ancestor on the African mainland, and occurring only
in Madagascar (Pozzi et al. 2014). They arrived in the island earlier
than other mammalian orders, resulting in a variety of adaptations to
diverse niches and feeding habits, which include the consumption of
fruits, leaves, flowers, nectar, bark and insects (Richard & Dewar
1991; Dewar & Richard 2012).
Previous work suggests that lemur colonization and subsequent
diversification may have been facilitated by the divergence of endemic
Malagasy angiosperms. Based on physiognomic and chronological
reconstructions, Génin et al. (2022) concluded that the generalization
of mutualistic interactions (such as frugivory and nectarivory) might
have boosted the coevolution of modern lemurs and at least 10 new
angiosperm families. However, observations were based on physiognomic
and chronological reconstructions, and empirical evidence remains
elusive both for pattern and process. Also, we still lack clear evidence
for the potential of mutualistic and antagonistic interactions in
generating congruence in the phylogenies of lemurs and Malagasy
angiosperms. Here we address these issues by testing for cophylogenetic
signals across a spectrum of plant-lemur interactions, including
frugivory, folivory, florivory, granivory and nectarivory. To support a
role for coevolutionary processes, we first expect phylogenies of lemurs
and plants to be congruent. This is to be expected if mutualistic and
antagonistic interactions shape reciprocal evolution regardless of
interaction type. We also expect to identify putative coevolving
phenotypic traits that may have propelled diversification (Blasco-Costa
et al. 2021). If both predictions met, we finally expect to find a
positive correlation between phylogenetic distances and dissimilarities
in sets of interacting partners for both lemurs and plants. This would
help to rule out the possibility that the observed cophylogenetic
patterns were driven by phylogenetic tracking or vicariance instead of
coevolution (Kahnt et al. 2019).