PhD Defensio

18.06.2020

Mag. Sandra Kirchner

Shedding light on the evolution of an enigmatic fish. Comparative analyses of cavernicolous and surface-dwelling populations of Garra barreimiae.

sandra.kirchner@nhm-wien.ac.at

Advisors: Harald Krenn, Luise Kruckenhauser (NHM)

PhD Defensio - Thursday, June 18th

Summary

The climate in the southeastern Arabian Peninsula is hot and dry, and in the course of global warming the large-scale desertification in this region and the associated water scarcity continues to progress. Despite these unfriendly conditions, the fauna of this geographical region comprises also freshwater organisms like the cyprinid Garra barreimiae. This fish can be found in all kinds of water bodies, in the few permanent lakes and streams as well as in countless temporary waters throughout its distribution area in the Hajar mountains in northern Oman and the UAE. The harsh conditions under which this fish has managed to sustain present it as a true survivor who outlasts long periods of drought, probably by retreating into the water reservoirs in the numerous karst caves of the Hajar mountains temporising/holding out for the next rainfall.

In 1980 Dunsire & Gallagher discovered an unpigmented and blind cave population in the underground karst lakes of the Al Hoota cave in northern Oman. With a total length of approx. 5 km the Al Hoota cave is the largest cave in the east of the Arabian Peninsula and comprises four underground lakes. The cave can be reached through one of the two entrances. The fact that there is a height difference between these two entrances transforms the cave into a flow-through system with up to 100 m3/sec water flushing through the cave in case of heavy rainfall. Due to the great morphological similarity to the surface form, the cave form was identified as G. barreimiae (besides the obvious differences such as the absence of eyes and pigmentation). Interestingly, the larvae of the cave form possess eyes after hatching, which are overgrown by surrounding dermal tissue in the course of ontogenetic development until no externally visible structures are present in the adult individual. Given that these troglomorphic characters are still variable, this leads to the assumption that the Al Hoota population is of rather young age.

The present PhD thesis deals with the species (or species complex) of G. barreimie and the genetic diversity throughout its distribution area, presenting extant population structures between the countless populations, some of which are separated by geological barriers. Of special interest is how genetically distinct the two troglomorphic cave populations are from the nearby surface populations. Furthermore, the present PhD thesis investigated the prevailing distribution patterns and morphological variations in the context of harsh and unsteady environmental conditions.

The project started with developing and establishing adequate primers for the analysis of nuclear microsatellite markers. Next-generation sequencing (NGS) data was generated via 454 sequencing, suitable primer regions were extracted and tested extensively in the wetlab. After successful establishment of 19 variable microsatellite primers (Paper 1), these were used for population genetic analyses of selected populations and the results suggest that the cave-dwelling form is genetically differentiated from the nearby surface populations, but occasional gene flow from the cave to the surface populations occurs. These findings show that although the cave population is genetically isolated, hybridisation between the two morphotypes is still possible and occurs under certain circumstances (cave habitat).

Further comparative studies on the development of the visual system (eye, optical nerve, lobus opticus, retina, lens...) between both morphotypes and the effect of absence/presence of light during development are already underway. Modern imaging methods as µCT and histological methods are used to quantify the degree and variation of reduction of the visual apparatus in different developmental stages.

The next step was to gather data on the genetic variation within the distribution range of G. barreimiae. This was accomplished by analysing three mitochondrial genes as well as applying the previously established nc microsatellite markers on a broad scale. The data allowed us to identify a total of five clearly separated groups whose genetic distances are comparable to other species of the genus Garra. The occurrence of these groups is geographically separated, mostly by geological barriers such as deep valleys (wadis) or mountain ranges. One of these groups was previously valid as a subspecies of G. barreimiae (G. b. gallagheri, Krupp (1988)) and was recently elevated to species (G. gallagheri). The remaining four groups were named according to their geographical distribution: North, East, West and Central. These findings raised the question whether the genetic groups can also be differentiated morphologically. In the course of an MSc thesis, extensive morphometric measurements were carried out and evaluated. Although different trends of individual characters could be identified, a clear differentiation and assignment solely on the basis of measurements and meristics is not possible. The fact that we are dealing with several, genetically clearly separable species that cannot be distinguished based on morphology and have so far been treated as one species justifies the term "cryptic species" (according to Bickford et al. 2007). The review and description of the newly discovered species are already in preparation and will include two already known names (West - G. barreimiae, Central - G. longipinnis) along with two new species names (East - G. sharqa, North - G. shamal).

The fact that this system comprises countless surface populations in addition to the troglomorphic Al Hoota cave morphotype enables a multitude of comparative studies on evolutionary adaptation to life under extreme conditions and the associated modifications. So far, only two isolated troglomorphic cave populations of this species are known, both occurring in neighbouring underground karst lakes. With such a limited occurrence, a population is always exposed to the threat of disappearing completely as consequence of single deleterious events (chemical pollution, desiccation, etc.). In light of the fact that the putative species G. barreimiae is classified as "Least Concern" in the IUCN red list yet comprises several cryptic species increases the pressure to re-evaluate the conservation status of these species individually. Progressive desertification, habitat destruction and water scarcity are factors that pose additional threats to a species living under harsh environmental conditions, especially if their occurrence is geographically restricted. The existence of both morphotypes enables comparative studies which focus on exploring the effects of alternative evolutionary pathways. Hence, this system represents a valuable contribution to the scientific community.