Significant genetic divergence and low morphological
differentiation between Cycas bifida and C. micholitzii
Previous phylogenetic analysis on major clades of the core Cycassection Stangerioides revealed that C. bifid a and C.
micholiztii were not sister species and indicated that the dichotomous
leaflet morphology appeared to have evolved independently several times
in the genus of Cycas (J. Liu et al., 2018). However, the two
species were sister species according to the phylogeny of transcriptome
data, which may be related to leaf material selection (Y. Liu et al.,
2022). It is because they have similar pinnate bifurcation patterns that
transcriptome data show that they are closely related. We tend to
believe the previous view (J. Liu et al., 2018). Similarly, our results
provide strong evidence that the two leaflet dichotomous species had
significant genetic divergence based on DNA sequences and microsatellite
datasets. Except for nuclear genes AAT and SAMS , two
species-specific groups of haplotypes were recognized in the Network
analyses (Figure 2). Evidences can also be found from the genetic
differentiation coefficient F ST, which is greater
than 0.15 and indicates that significant genetic differentiation exists
in the two species (Table 3). Both the results of STRUCTURE and PCoA
showed that populations were grouped into two clusters corresponding to
the two species (Figure 3a, b).
We attribute this low morphological differentiation and high genetic
divergence between the two species to two possible hypotheses. The first
one, the differentiation of the two species is caused by long-term
geographical isolation. The distribution areas of these two species are
separated by the RRFZ which can block the gene flow of populations or
species on its both sides (Y. Q. Gong et al., 2015; Yang et al., 2017;
Y. Zheng et al., 2016). The Indochina Plate moved about 700 km southeast
relative to the South China Block along the Red River fault line dued to
the collision of the Indian block with Asia (Tang, 2004). The movement
of landmasses along the Red River Gulf likely caused many formerly
closely related populations of Cycas to separate from one
another, stimulating the production of new species through subsequent
isolation (Tang, 2004). Cycas bifida and C. micholiztiimay have been one species once or from the same common ancestor, and
they are separated species now with high genetic divergence due to the
long-term geographical isolation caused by the tectonic movement along
the Red River Gulf. The other, their morphological similarity, such as
the pinnae bifurcate in same way, is only the result of convergent
evolution.
Of the two possible causes, we tend to believe that it is the long-term
geographic isolation with the subsequent local adaptation that caused
the two species to differentiate. Because in our study, the two species
have significant IBD based on the mantel test (Figure 6c) and very
little gene flow based on IMa analysis (Figure 5b), and there is a
significant barrier between their distribution areas according to the
Barrier analysis (Figure 3c), indicating that they diverged by the
geographic isolation. According to Hill, species of Cycas that
exhibit morphological features similar to those of phylogenetically
related species may hybridize freely with each other (Ken D Hill,
Stevenson, & Osborne, 2004)(Hill et al., 2004; Hill, 2008). Therefore,C. bifida and C. micholitzii may have gene flow or gene
introgression with the Cycas species in the adjacent region,
which results in closer phylogenetic relationship with them. Convergent
evolution, on the other hand, is unlikely. In many species ofCycas , it is unlikely that only two of them have such a
consistent pattern of pinnate branching, or the pinnate branching cannot
be used as the taxonomic character for classification of Cycas .
The simulated niche similarity results also showed that niches of the
two species were not overlapping and were not identity (Figure 4), which
further indicated that convergent evolution couldn’t happen between
them. In the future, comparative genomic studies on Cycas species
will help to reveal why only the two species have the same pinnate
bifurcation pattern among so many Cycas species, and that were
possibly important in their origin.