Results
Our results show reproductive character displacement (RCD) between
sympatric species of Ruellia relative to allopatric species (Fig.
3). Overall floral form is significantly more different between
sympatric species pairs than between allopatric species pairs
(Ranosim = 0.63, p = 0.017). Sympatric species
were not significantly more different for overall leaf form
(Ranosim = 0.38, p = 0.121). When examining
individual floral characters, four of these showed a significantly
greater difference between sympatric species pairs (Fig. 3), with style
length showing the most pronounced difference (Fig. 3;Ranosim = 0.85, p < 0.001). In
contrast, only one leaf character appears to show significantly greater
differences between sympatric species pairs relative to allopatric
species pairs (leaf length; Ranosim = 0.54, p =
0.036). If conservative Bonferroni corrections are applied to these
multiple tests of significance, only style length shows a significantly
greater difference between sympatric species pairs relative to
allopatric species pairs.
Among 95 total crossing attempts across five sympatric species pairs,
only one instance was successful and yielded mature, viable seeds (a
single cross between Ruellia conzattii and R.
hirsuto-glandulosa ). In contrast, 63 of 730 crossing attempts between
allopatric species pairs yielded mature viable seeds (Supplementary
Table 2). Statistical analyses using generalized linear mixed models to
control for the non-independence of data points involving the same
species (following Tobias et al. 2014) showed that occurrence in
sympatry significantly reduced crossing success of species pairs (Table
1; likelihood ratio-test, Χ2 = 5.00, d.f. = 1, p =
0.025). This result is supported by a non-parametric Wilcoxon test on a
phylogenetically corrected dataset (W = 6, p = 0.032).
Using the same generalized linear mixed model approach, we found that
pairs of species with similarly shaped and similarly colored flowers had
significantly higher crossing success (Table 1; Fig. 4; flower color:
Χ2 = 18.94, d.f. = 1, p < 0.001; flower
shape: Χ2 = 5.76, d.f. = 1, p = 0.016; multivariate
depiction of floral morphospace provided in Supplementary Fig. 2).
Meanwhile, similarity in vegetative morphology did not significantly
influence crossing success (Table 1; Χ2 = 2.91, d.f. =
1, p = 0.089). We also found that more distantly related species pairs,
quantified as interspecific genetic distance in a maximum likelihood
phylogeny, had significantly lower crossing success (Table 1; Fig. 4;
Χ2 = 4.29, d.f. = 1, p = 0.038). Similar results with
respect to significance of fixed effects were obtained when testing the
effect of time since divergence in a temporally calibrated phylogeny
(Supplementary Table 3). In a multivariate analysis of drivers of
crossing success across all species pairs, we found the same direction
for our fixed effects as in univariate analyses, but floral shape
similarity and geographical range overlap of species pairs did not
significantly modulate crossing success (Table 1). An assessment of our
model showed these two fixed effects to be highly correlated (r = 0.37)
whereas none of the other fixed effects were highly correlated with each
other (r < 0.17). If either geographical range overlap of
species pairs or similarity in floral shape was removed from the model,
the model performed better in explaining crossing success (Table 1;
ΔAICc after removing geography = 1.3, ΔAICc after removing floral shape
similarity = 0.9), but if both were removed, the model performed worse
(ΔAICc = -2.5). This may be expected given the documented reproductive
character displacement in sympatric species pairs.