Wing color pattern evolution
During the course of their evolution, Prepona underwent drastic
changes in the color pattern of both wing surfaces (Figures 2-3). These
color patterns were the key features used by previous taxonomists to
classify Prepona species in two genera. The former genusAgrias contained yellow, orange, blue and red species andPrepona , as delimited previously, contained mostly blue species.
The former group being nested within the latter is supported by
molecular and morphological data
(Ortiz-Acevedo et al., 2017), while
color patterns apparently transitioned rapidly from blue to red (Figure
2), resulting in Prepona containing butterflies with such
strikingly different coloration patterns (Figures 1-3).
Our results suggest that there is stronger change in phenotype for red
and blue RGB channels (Table 3) than for green and total RGB. The
changes identified are mostly jumps in the mean and are located around
the red Prepona clade (Figure 2). The analysis consistently
identified the jump from blue to red in Cell 1 with no shift in the rate
of evolution. Despite the fact that the red Prepona clade is so
notably different in color pattern compared to the rest of the tribe’s
members, the evolutionary rate of phenoptyic change after the jump
remained similar to that in blue clades. These jumps are consistent with
previous findings, that drastic changes in color patterns of butterfly
wings and other organisms have a relatively simple genetic basis and can
appear relatively fast in evolutionary time
(Nadeau et al., 2016; Reed et al.,
2011). Differential genetic expression in particular regions of the wing
are also responsible for localized changes
(Brakefield et al., 1996; Nadeau et
al., 2016; Oliver et al., 2012), which might be a plausible explanation
for the results we found in Cell 1. These localized changes are not
always visible under analysis of the entire wing, thus, isolating those
regions that are likely to change quickly allowed us to detect the jumps
in phenotype.
Jumps and shifts in the blue channel are restricted to the tips of the
phylogeny and might be associated with recent speciation events. For
example, we identified two jumps in the blue channel of Cell 2 (Figure
2); i) in the branch leading to Prepona amydon+P. hewitsonius and
ii) in P. hewitsonius . These species are phenotypically extremely
diverse in forewing coloration, ranging from dark, deep blue to light,
bright blue, even within subspecies (Figure S7). We also identified a
change in the Cell 3 blue channel at the branch leading to Prepona
philipponi (W) . Prepona philipponi (W) has a different
light blue tone compared to P. philipponi (E) (Figure S8), but
with only a single studied individual of the former taxon, this result
needs corroborating with further samples.
A potential explanation for the dramatic change of color patterns in
Preponini is involvement in mimicry rings with genera such asCallicore and Asterope (Nymphalidae, Biblidinae), which
have remarkably similar color patterns on both wing surfaces
(Descimon, 1977; Jenkins, 1987;
Figure S9). The inferred origin of the red clade in Preponapartially supports this hypothesis, since both Callicore andAsterope are predominantly Andean and Amazonian groups. In
particular, the red Prepona showed no change in the Amazonian
ancestral range as it underwent speciation (Figure 3). Mimicry is
plausible since species in both groups feed on toxic plants, including
the families Sapindaceae and Erythroxylaceae, but whether they sequester
plant toxins as caterpillars and retain them after emerging as adults is
still unknown, as is their palatability. Preliminary geographical
distribution data show a correspondence in coloration and distribution
among Prepona , Callicore and Asterope(Ortiz-Acevedo et al., in prep.).
Diversification of
Preponines
In studies of other Neotropical butterfly groups, natural history traits
and paleo-climatic events have been shown to shape diversification rates
by influencing speciation and/or diversification
(Chazot et al., 2020; De-Silva et
al., 2016; Sahoo et al., 2017). This seems to be the case for Preponini
as well. Our likelihood-based diversification rate analyses suggest a
constant diversification rate over 30 Ma and is mostly congruent among
alternative methods (Appendix). The diversification rate estimated by
our likelihood analysis (0.12) is within the range of diversification
rates estimated for other butterfly groups
(Chazot et al., 2020; Peña and
Espeland, 2015). However, we found support for exponentially varying
speciation rates, similar to speciation rates in Neotropical Nymphalidae
(Chazot et al., 2020). Extinction in the three top candidate models wasa priori fixed to zero, as estimated for larger Neotropical
butterfly clades during the same geological time
(Chazot et al., 2020).
Our analyses support an exponential increase of speciation promoted by
the Andean uplift (Table 1). In fact, this biogeographically dependent
model has slightly higher support than the time dependent models
discussed above. A similar result has been found in many other
organisms, both lowland and highland, and is hypothesized to result from
the ecological opportunity created by emergence of new niches and the
restriction of dispersal across the Andes (Smith et al 2014). Our
ancestral range estimates show that, after the two last pulses in Andean
uplift, lineages mostly contracted in their ranges leading to allopatric
sister clades. For example, the ancestor of the clade containingArchaeoprepona licomedes , A. priene and A. chromusmost likely had an eastern Andean distribution, while its sister clade
had a probable Central American ancestor (Figure 3). Also, the clade
containing all Prepona species except P. dexamenusoriginated from a Chocoan ancestor, and one daughter lineage retained
its ancestral range while the other dispersed to the Amazon, Guianas and
Atlantic forest (Figure 3). The origin of many recent Preponini species
dates to a time of intensified Andean uplift, in which the Pebas system
started to drain and the Amazon river reached its current configuration,
aided by the formation of the Acre system
(Hoorn et al., 2010). The
Miocene-Pliocene boundary was also characterized by a global vegetation
change (Cerling et al., 1997).
A recent study showed that phylogenetic trees alone do not have enough
information to tease apart different evolutionary scenarios since
particular topology can either result from high speciation and constant
extinction or decreased extinction and constant speciation (Louca and
Pennell, 2020). Consequently, we would expect similar support for those
alternative scenarios. Here we show however that the support for models
with constant speciation and no extinction or exponentially increasing
speciation with no extinction is considered to be much stronger than the
models with constant speciation and decreasing extinction under an
evidential statistics framework (Taper and Ponciano, 2016).
Origin and Biogeographical
Patterns
Although the high Amazonian species richness of the tribe would suggest
an out-of-the-Amazon biogeographical model, the tribe Preponini
originated from a widespread South American ancestor at
~27.5 Ma in the early Oligocene. By this time, South
America, dominated by forests
(Strömberg et al., 2013), detached
from Antarctica and moved north towards North America
(Axelrod et al., 1991). This dating
is consistent with other studies that used independent datasets
(Peña and Wahlberg, 2008; Wahlberg
et al., 2009). Members of Preponini likely dispersed, colonized and
diverged in new niches in Central America as the bridge between land
masses became more continuous at ~23 Ma
(Bacon et al., 2015; Montes et al.,
2015). This event has been demonstrated to have played a major role in
the diversification of multiple groups of organisms
(Bacon et al., 2015).
It has been suggested that some of methods for ancestral range
reconstruction are biased towards estimating widespread ancestral ranges
(Buerki et al., 2011; Clark et al.,
2008; Matzke, 2014; Ree and Smith, 2008), as we found in the ancestor of
Preponini. To reduce this bias, we restricted the ancestral estimate to
contain only six of the eight areas considered, as observed in the most
widespread extant Preponini species (Ronquist and Sanmartin, 2011, i.e.Archaeoprepona amphiachus, A. demophoon, A. demophon ). In
addition, climatic changes accompanying the evolution of Preponini make
biological sense in light of our inferred ancestral range estimate, as
we discuss below.
Early speciation events in the tribe are characterized by range
maintenance (~13 Ma; Nodes II and III; Figure 3).Subsequently, the ancestor of Archaeoprepona contracted in range,
becoming restricted to the Atlantic forest region and subsequently
dispersing to the Chocó. The transition from Atlantic Forest to Chocó
happened through an anagenetic expansion including all South American
areas except Western Andes and a subsequent range contraction (Table
S15). The restriction of the ancestor of Archaeoprepona to
Atlantic forest is coincident with a period in which tropical forests in
South America were likely reduced by a decrease in global temperature
(Kürschner et al., 2008; Pound et al., 2011). The late Miocene was
probably characterized by grassland habitats, as suggested by high
diversification of hoofed animals and changes in their dentition
(Kürschner et al., 2008). Preponini species are not currently known to
inhabit these grassland habitats. The subsequent dispersal to the Chocó
follows an increase in global temperature and the reappearance of
widespread tropical forests
(Kürschner et al., 2008; Pound et
al., 2011).
After the colonization of Chocó tropical forests, Archaeopreoponadispersed to central America at ~8 Ma during periods of
intensified biological migration between Central and South America
(Bacon et al 2015). From this point forward, high geologic activity in
South America during the Pliocene
(Hoorn et al., 2010) likely promoted
dynamism in current ranges in which some species became widespread
(clades containing Archaeoprepona demophoon and A.
amphimachus ), while other became restricted (e.g A. chromus andA. priene ) to small geographic areas.
Conversely, Prepona +Mesoprepona , which originated from a
South American ancestor, shows contrasting patterns of range evolution.Mesoprepona , a monotypic genus, shows a reduction of its
distribution range and became restricted to eastern South America. In
contrast, the relatively species-rich Prepona shows early
diverging clades to have dispersed to a larger area, including South and
Central America, around a time of increased migration between these two
continents (Bacon et al., 2015). The
early branching Prepona laertes clade was found to be initially
restricted to eastern South America, with subsequent dynamism in range
contraction/expansion as it underwent speciation events. More recent
clades were found to have contracted their ranges at ~7
Ma, becoming restricted to the Chocó region, while the Amazon region
suffered landscape reconfiguration resulting from the transition of the
Pebas system to the Acre system.
The subsequent speciation events show: i) range maintenance in the
restricted Prepona werneri , ii) range expansion followed by
contraction in the P. pylene and P. deiphile clade, and
iii) slight range expansion followed by contraction in the redPrepona clade (Node III, Figure 3). The dynamic range evolution
in these clades happened in the early Pliocene ~5 Ma,
which is characterized by landscape reconfiguration due to a strong
activity in Andean uplift (Hoorn et
al., 2010; Figure 3). This intensified final uplift of the Andes likely
allowed the earliest divergent red Prepona to shift from
lowland-highland distribution and become restricted to higher elevation
habitats, while the ancestor of the remaining red Prepona species
became restricted to the Amazonian lowlands. Current distribution ranges
of the taxa in this clade show expansion of their ranges to eastern
South America and dispersal to habitats west of the Andes.
Other examples of the influence of the Andean uplift on speciation and
range dynamics of Prepona clades is exemplified in the clade
containing P. pylene, P. eugenes, P. gnorima and P.
deiphile. Recent phylogenetic reconstructions and morphological
analyses of the tribe split the previously widespread Prepona
pylene (sensu Lamas, 2004) intoP. pylene, P. eugenes and P. gnorima . Our dating suggests
that Prepona gnorima, distributed in Central America and Chocó,
split from the eastern South American P. pylene +P. eugenesat ~5 Ma. Furthermore, Prepona deiphile , although
currently considered a widespread species, is paraphyletic in our
phylogeny, supporting a split into a Central American and South American
clade(Ortiz-Acevedo et al., 2017; Turrent Carriles and García Días,
2019) that probably happened during the last pulse of Andean uplift.
Preponini genera exhibit contrasting biogeographical histories,
suggesting that they might have evolved under different evolutionary
pressures and scenarios. Differences in natural history, and in
particular larval host plant relationships, potentially underlie these
differing biogeographical patterns as demonstrated in the closely
related Neotropical charaxine tribe Anaeini for instance (Toussaint et
al., 2019). Unfortunately, knowledge of host plants in Preponini is
still rather incomplete, although, researchers, collectors and
enthusiasts are working together to fill this gap in knowledge.
Conclusions
This first investigation of color pattern evolution in Preponini posits
new hypotheses for the observed shifts across the phylogeny of the
group. We found that, contrary to what might have been expected, changes
in wing color did not influence diversification rates in this group.
Both the formation of the Isthmus of Panama and the uplift of the Andes
mountain range positively impacted the diversification of Preponini.
Landscape reconfiguration and ecological opportunity created by the
complex emergence of the connection between Central and South America
and mountain building allowed dispersal, colonization and divergence in
newly available niches. Further studies should focus on the mechanisms
that triggered the striking change in color that happened rapidly in the
genus Prepona . In particular, Prepona are potentially
involved in mimicry rings with Callicore and Asterope ,
which also show a high richness in the Amazon region, with a number of
species restricted to this area.