Reconstruction of population size over time using the EBSP approach
indicated that C. obesa in the ECS experienced significant
population size changes. After a prolonged stable period, the ECS
population decreased from approximately 20000 years ago and increased
from approximately 2000 years ago (Figure 4). The subsequent
goodness-of-fit tests also supported the null hypothesis of the sudden
expansion model, with non-significant values for SSD.Figure 4. Historical population size change of C. obesain the ECS based on EBSPs
4. Discussion
In the present study, we used the mitochondrial sequence COI to examine
the genetic diversity and phylogeographical structure of the sea penC. obesa in the ECS. Genetic variation among the ECS populations
was generally low, perhaps due to a combination of high contemporary
gene flow and the recent common ancestry of haplotypes. Larvae of sea
pens, called planulae, usually drift freely for approximately 1 week
before settling in sediments(López-González et al., 2000). If
appropriate sediments are not available, settlement can be delayed by up
to 1 month. The long planktonic larval stage in C. obesa may
facilitate gene flow by current-driven dispersal of pelagic larvae, and
consequently decrease the genetic structure among distant populations in
the ECS.
Ocean currents play an important role in the genetic flow connections
between populations(York et al., 2008). In summer, the ECSCC enters the
ECS from the South China Sea and the YSCC from the Yellow Sea. These
prevailing currents transport large numbers of planktonic larvae across
the marginal seas of the northwest Pacific. The CDW has been reported to
act as a physical barrier for some marine species. However, the
influence of the CDW on population differentiation is taxonomically
variable; some species show no genetic breaks between populations from
each side of the CDW(Ni et al., 2017). Species with a long planktonic
larval stage usually show substantial mitochondrial homogeneity across
the CDW, whereas species with a sessile life history are more prone to
biogeographic and historical barriers. The genetic structure of C.
obesa in the ECS seems to be uninfluenced by the CDW and may benefit
from the long planktonic larval stage.
Mismatch distribution analyses indicated that the ECS population
recently experienced a bottleneck, which may have resulted in low
genetic diversity. Rare variants in small populations are predicted to
be eliminated through genetic drift. Recent population isolation and
fragmentation during the Pleistocene glacial age increased the role of
genetic drift in COI variation in C. obesa(Luo et al., 2012). The
mitochondrial gene evolution rate in sea pens is relatively
slow(Williams, 2011), which causes them to exhibit lower genetic
diversity than other species at the same time. We speculated that the
mitochondrial genetic structure of C. obesa in the ECS may have
retained the genetic features of the historical population during the
Pleistocene.
The Pleistocene glacial age, particularly the last glacial maximum
(LGM), approximately 20,000 years ago, has had an important influence on
the evolution and genetic structure of marine organisms. Some marine
species appear to have undergone dramatic population expansion during
the LGM when the sea level fluctuated. The population size of C.
obesa decreased rapidly when sea level decreased and the ECS shrunk.
When sea level rose, C. obesa experienced demographic expansion
and reoccupied the seabeds reflooded by the ECS. In that case, the
theoretical genetic homology of C. obesa populations in the ECS
was high, which is consistent with our results.
Most phylogeographic studies in the ECS have focused on economically
important species such as fish and mollusks, and more comparative
phylogeographic studies using additional species pairs are needed to
explain the underlying mechanism of these discrepancies. Our results
imply that sea pens such as C. obesa are good candidates for such
comparative studies, as they are abundant in this region and suffer less
from human-mediated activities than commercially exploited fish and
mollusks.Funding: This research was funded by the National Key Research
and Development Program of China (grant number 2021YFC3101702).Data Availability
Statement: The data that support the findings of this study are
available on request upon reasonable request.Acknowledgments: The authors thank Wang Hangjun for collecting
the samples.Conflicts of Interest: The authors declare no conflicts of
interest.
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