INTRODUCTION
Elucidating the factors contributing to population divergence and
speciation is the core question in evolutionary biology and has
attracted interest from biologists for a long time (Darwin, 1859;
Dobzhansky, 1982; Mayr, 1947; Turelli, Barton, & Coyne, 2001).
Speciation is a gradual process occurring through the cumulative action
of natural selection and genetic drift in which populations diverge into
groups that cease or limit allele exchange (Coyne & Orr, 2004; Futuyma,
1998; Raman, 1998). For example, some models of speciation with gene
flow suggest that reproductive isolation (RI) begins at a single locus
but will progressively expand and facilitate divergence of other linked
genes throughout speciation (Rieseberg & Blackman, 2010; Wu, 2001).
The narrowly differentiated peak in the genome can arise at multiple
prezygotic and postzygotic life-history stages (Coyne & Orr, 2004;
Haerty & Singh, 2006; Presgraves, 2010). During population divergence,
prezygotic isolation mechanisms are more important than postzygotic
barriers because they act earlier before hybrid zygotes are formed.
Additionally, morphological divergence is expected to be associated with
prezygotic reproductive barriers. When taxa begin to evolve
morphological differences, they can directly or indirectly reduce
mattings, sperm transfer, or fertilization (Coyne, 2016; Feder et al.,
1994). For example, Turbek et al. found that assortative mating based on
both plumage coloration and song can maintain premating isolation
between Sporophila iberaensis and S. hypoxantha (Turbek et
al., 2021). This view is also applicable to plants. For many
taxa, including Aquilegia , flower color differences leading to
pollinator shifts likely contribute to RI between populations (Des
Marais & Rausher, 2010; Hodges, Whittall, Fulton, & Yang, 2002;
Kuriya, Hattori, Nagano, & Itino, 2015; Quattrocchio et al., 1999;
Schwinn et al., 2006). Additional differences between species that
result in further RI can also accumulate after speciation is already
complete. Furthermore, distinguishing between genetic changes that play
determinant roles in speciation and those that arise after speciation is
complete may be challenging. Thus, organisms in the early stages of
speciation provide an opportunity to identify causal differentiation in
speciation of the genomic landscape.
With the publication of new species, Aquilegia currently includes
approximately 110 species worldwide, with approximately 70 species based
on the record of Munz, and is widely distributed in the northern
temperate zone (A. S. Erst et al., 2020; A. S. Erst et al., 2017; Luo,
Erst, Yang, Deng, & Li, 2018; Munz, 1946). Lu et al. analyzed the
genomes and methylomes of 10 Aquilegia plants worldwide and
speculated that the specific genetic and epigenetic structure endowsAquilegia with a high degree of adaptability and promotes
adaptive radiation speciation of the genus (Lu et al., 2019). However,
these species are mature in the late stage of differentiation. In
addition to the gene regions that began to differentiate in the early
stage of speciation, various factors have led to obvious differentiation
of the genome among species. Unlike its congeners, the phenotypes of
different populations of A. viridiflora showed high diversity,
which occurs in the early stage of speciation. A. viridiflora is
an ideal material to study the key genes that lead to RI in the
early stage of speciation.
Aquilegia viridiflora shows rich morphological variation in
different populations, so Andrey S. Erst et al. identified A.
viridiflora with laminae purple or lilac-blue petals as the new speciesA. kamelinii (A. Erst, Shaulo, & Schmakov, 2013) and A.
viridiflora with dark purple petals in North China as the new speciesA. hebeica (A. S. Erst et al., 2017). However, the publication of
these new species lacks support from molecular data. According to field
surveys, the phenotypic variation of A. viridiflora is continuous
(see Figure 1). Therefore, A. viridiflora , A. kameliniiand A. hebeica were considered the same species (A.
viridiflora ) in our study. A. viridiflora is primarily
distributed in northern China, extending between 75.14-132.58° E and
33.24-53.28° N, in which the east-west span is wide, and the environment
changes gradually. These factors suggest that environment-mediated
divergent selection may be the driving force for the early
differentiation of A. viridiflora . Additionally, other species ofAquilegia are produced through ecological speciation, such asA. formosa and A. pubescens (Hodges et al., 2002;
Whittall, 2005). Therefore, this species provided a test case to reveal
the effects of adapting to different ecological environments in the
early stage of the speciation process.
In the present study, we collected population samples covering the main
distribution range of A. viridiflora for genomic analyses and
morphological characteristic measurements to explore the following: (1)
the morphological divergence during the early stages of speciation, (2)
the genomic basis of such phenotypic traits, and (3) the role of the
environment in maintaining population divergence. Answering these
questions will reveal the speciation process of A. viridifloraand the environmental factors that lead to its differentiation.