Introduction
Qinghai-Tibet Plateau (QTP), formed by the collision of the Indian Plate
with the Eurasian Plate beginning in Eocene ~50 million
years ago (Mya), is often deemed as the Roof of the world or the Third
Pole due to its average altitude of over 4, 000 meters and massive
glaciers (Harrison, et al. 1992;
Molnar, et al. 1993;
Yin and Harrison 2000;
Tapponnier, et al. 2001;
Molnar, et al. 2010). High absolute
elevation, high radiation, severe cold, and hypoxia are the main
characteristics of extreme environmental conditions on QTP, which
greatly threaten the survival of the local creatures, especially the
wild plateau fish (Li 1991). Originated
from a common ancestor Barbinae, Schizothoracine fishes (Teleostei:
Cyprinidae), which distributed extensively in the high-altitude streams,
rivers and lakes scattered throughout QTP
(Zan, et al. 1985;
Yue 2000;
Li, et al. 2013), evolve gradually and
become highly adapted to life in high-altitude environmental stress, are
spectacularly diverse in ploidy, physiological plasticity, and
morphological innovations, such as numbers of scales, pharyngeal teeth
and barbels (Zhou, et al. 2020).
Polyploidy can also be commonly observed in Schizothoracine fishes and
is regarded as a successful evolutionary transition of organisms to
these extreme environments (Zan, et al.
1985; Leggatt and Iwama 2003;
Wang, et al. 2016).
Gymnocypris przewalskii (Kessler 1876, NCBI: txid 369649,
Fishbase ID: 55381, Figure 1), otherwise known as the Przewalski’s naked
carp or scale-less carp is an endemic tetraploid Schizothoracine fish to
Qinghai Lake (3196 m above sea level, a.s.l.) in the northeast margin of
QTP (Wood, et al. 2007;
Xiong, et al. 2010;
Tian, et al. 2019). Relative to organisms
at higher altitudes, G. przewalskii is an important link in the
evolution of biological plateau adaptation. Qinghai Lake is the largest
closed-basin lake in China with no surface water outflows, owning to
blocking the channel from Qinghai Lake to Yellow River around the end of
the Middle Pleistocene (An, et al.
2006).
The lake not only has high-altitude environmental characteristics, such
as a low concentration of dissolved oxygen (4.16-6.08 mg/L) and
unyielding cold (average annual air temperature ~
-0.1℃), but also has its own unique characteristics of strong alkalinity
(carbonate alkalinity approximately 29 mM, pH 9.1-9.5) and a high
salinity level (approximately 13 ppt)
(Wang, et al. 2003;
Xu, et al. 2010;
Zhang, et al. 2010;
Fu, et al. 2013;
Cui, et al. 2016). Due to high
evaporative water loss, decreasing water supplies, and extensive water
diversion for agricultural use, the salinity and the alkalinity levels
are increasing by 7% and 0.5% per year, respectively
(Yao, et al. 2016) . Unlike most
stenohaline cyprinids, G. przewalskii undergoes an annual
spawning migration between the saline-alkaline Qinghai Lake and the
freshwater tributary rivers. From March to July, fish migrate to
freshwater rivers to spawn and, upon completion of reproduction, return
to the lake for the rest of the year
(Walker, et al. 1995). This transition
from river water to lake water causes profound changes for G.
przewalskii , which include an acid-base disturbance, a rapid rise in
osmolality, and a rise in the concentrations of all measured plasma ions
(Wood, et al. 2007;
Yao, et al. 2016). Although historically
abundant, the naked carp is facing collapse due to overfishing and
destruction of spawning habitat through dam building for irrigation, and
has been listed as a national class II endangered species and assessed
as being vulnerable (VU) in the new Red List of China’s Vertebrates
(Wang and Xie 2009;
Jiang, et al. 2016).
G. przewalskii is an important and unique animal model for
studying aquatic biological development, genetics, evolution and
physiology in highland aquatic areas. This unique environmental
adaptability is very important for the study of low-oxygen, high
saline-alkaline and low-temperature tolerance of high-altitude fish and
the long-term evolution mechanism of aquatic organism with tetraploid
genome structure in this extreme environment. We therefore chose to
sequence, assemble and annotate the genome and transcriptome of G.
przewalskii as an example to reveal geographic isolation of
Schizothoracine fishes in Qinghai-tibet plateau lift and understand
evolutionary adaptation of G. przewalskii to high altitudes,
stressful saline-alkaline water environment.