4.1 Homostyles replace short-styled individuals within
populations
The variation in the frequency of floral morphs among populations
provides an opportunity to elucidate how homostylous phenotypes
originate and spread. The homostyles of Primula vulgaris ,
typically characterized by long styles and high anthers (Huu et al.,
2016; Li et al., 2016), display the stigma type of LS-morph but retain
the anther position and pollen type of the SS-morph (Crosby, 1940).
Therefore, the origin of HO-individuals from SS-individuals should cause
an initial reduction of the latter, triggering downstream consequences
that further increase the number of homostyles. Firstly, pollen of
homostyles can be used for both self- and cross-fertilization of
LS-individuals, but not for fertilization of SS-individuals (Figure 1).
Homostyles can also be fertilized by compatible pollen of SS-morphs, but
this cross is thought to occur rarely in nature due to stigma clogging
with self-pollen in homostyles (Crosby, 1959; but see Bodmer, 1959).
Hence, homostyles have a reproductive advantage over both LS- and
SS-morphs as a result of their ability to both self-fertilize and cross
with LS-morphs (Richards, 2003). Finally, self-fertilization of
homostyles (Figure S4A and B) and occasional crosses from homostyles to
LS-individuals produce homostylous and LS-progeny, but no SS-progeny
(Figure S4C and D). In accordance with these expectations and findings
of previous studies
(Crosby, 1949,
1959; Curtis & Curtis, 1985), our results demonstrate that, as
homostyles increase within populations, more SS- than LS-individuals are
lost within populations (Table 1 and Figures 2A and S1). The fact that
LS-individuals are maintained at low frequencies within populations,
even in the absence of SS-individuals, suggests that they are
occasionally fertilized by homostyles. Taken together, these results
corroborate the hypothesis that long-homostyles originate from
SS-individuals and explain why the latter occur at lower frequencies
than LS-individuals across populations with all three morphs.