Laszlo ORBAN, Ph.D, Director, SRP
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Laszlo Orban obtained his university diploma (1981) and doctoral degree (1983) at the Jozsef Attila University (now Szeged University) in Hungary. He spent his postdoc periods in the laboratories of Janos Nemcsok (JAU) and Andreas Chrambach (NIH), respectively. In 1989 he established the first fish molecular biology lab of Hungary at ABC (Godollo) and led it for ten years. In 1997 he received a Candidate of Biological Sciences degree from the Hungarian Academy of Sciences. From 1998 until 2002 he was a Principal Investigator at the Institute of Molecular Agrobiology, since then he has been leading the Reproductive Genomics Group at TLL. Dr. Orban is an Adjunct Professor of the Georgikon Faculty at the University of Pannonia (Keszthely, Hungary) and an Adjunct Associate Professor of the Department of Biological Sciences of the National University of Singapore. You may wish to contact A/Prof Laszlo ORBAN at: |
For information on PhD studies at TLL, click HERE Research Interests
The main interest of our team is to understand more about the genetic regulation of fish sex. Accordingly, most of our research projects are aiming to answer questions related to various aspects of teleost reproduction by using the tools of molecular biology, genetics and functional genomics.
Our main research projects are the following:
1) Comparative studies on sex determination and gonad differentiation in teleosts
The genetic regulation of sex is poorly understood in most teleosts. We are studying the genes involved with teleost sex determination and gonad differentiation in commercial species and in the zebrafish (Danio rerio) model.
Despite of the extensive knowledge available about most aspects of the biology of zebrafish (Danio rerio, Cyprinidae), the primary teleost model, the molecular regulation of its sex is virtually unknown. From a series of experiments performed with classical (e.g. genetic selection through several generations of multifactorial crosses, genome manipulations, etc.) and modern tools (e.g. comparative genome hybridizations) we have obtained data that strongly indicated polygenic sex determination in the species. We are currently building platforms allowing for the identification of factors (genes) involved in the primary decision during the determination process.
Zebrafish are juvenile hermaphrodites. Accordingly, all the males go through a ‘juvenile ovary stage’ before their gonads start to transform into a functional testis albeit at different time points and with different level of commitment. By studying the transcriptomes during gonadal transformation (32-36 dpf) using a home-made cDNA microarray, we have shown the involvement of a major signaling pathway in the process. The up-regulation of the pathway through treatment with a small-molecule chemical we were able to affect the sex ratios substantially, providing functional evidence for the involvement of the pathway in the process.
The data expected from this project will help us to understand not only the gonad differentiation of zebrafish, but also that of the related, commercially important farmed cyprinid species. Moreover, as the zebrafish gonad during its transformation from ‘juvenile ovary’ to testis shows morphological features similar to those in the transforming gonad of protogynous hermaphrodites, it could be a good model for the study of the sex reversal process in those species as well.
2) Understanding the process of gonad transformation in marine sex changers
In collaboration with the Yue group (TLL) and the Agri-food and Veterinary Authority (AVA) we are working on the development of superior stocks of the Asian seabass or barramundi perch (Lates calcarifer) through marker-assisted selection. Over the years, our team has generated a number of platforms and collected valuable data from the species. One of the main problems of Asian sebass culture is due to the species being a protogynous hermaphrodite, resulting in constantly changing sex ratios in the broodstock. We have collected blood and tissue samples from juveniles, young adults and fully mature adults and analyzed them with a selected set of biochemical and molecular markers. We have found several markers that can be used to determine the maturation and reproductive status of brooders in a non-invasive manner. Data from these markers and systematic manipulation of keeping conditions will provide us better understanding and control of the ‘male-to-female’ gonad transformation process in the species. We have also adapted a cryopreservation protocol and collected sperm samples from several dozen male brooders.
3) Unraveling the secrets of the reproductive behavior and evolution of Asian arowana
We have a pioneering research collaboration with Qian Hu Fish Farm Corp. (Singapore) on the reproductive biology of a primitive teleost, the Asian arowana or dragonfish (Scleropages formosus). The Asian arowana is regarded as one of the most valuable ornamental fish species and it is protected by CITES since 1980. Over the past twelve years, we have produced data on the genetic diversity of broodstocks and natural populations, molecular identification of color variants and sexes, and the mitogenome of the species. Currently, we are working on a set of questions related to mate choice and parental care, by using ‘state-of-the-art’ molecular techniques. We are also trying to produce a genetic linkage map and a molecular toolset that can be used to obtain information about the reproductive and general health status of arowana brooders.
4) Improving aquaculture practices with the application of modern, molecular tools
One of the problems of modern aquaculture is the lack of non-invasive tools allowing for the differentiation of the two sexes in species lacking sexual dimorphism in adults. We have adapted a set of technologies and developed others for the identification and characterization of polymorphic DNA markers potentially suitable for molecular sexing of commercially important and model teleosts.
Sex-specific and sex-associated DNA markers have been isolated from a number of species, including African catfish, Asian arowana and the turbot. Other searches have indicated the lack of major differences between the male and female genomes in green spotted pufferfish and zebrafish.
We have also developed methods for the molecular identification of color variants of Asian arowana and two phenotypically similar rockfish species. Our current efforts are being focussed on the extension of toolset to microarray-based approaches.
5) Studies on the genetics of scale pattern formation in cyprinid teleosts
We are studying the genetics of scale pattern formation in cyprinids, in collaboration with Prof. Miklos Bercsenyi (Univ. Pannonia, Keszthely, Hungary) and Drs. Matt Harris and Nick Rohner (Max Planck Institute for Developmental Biology, Tubingen, Germany). Seventy years ago, Kirpichnikov and colleagues have crossed common carp varieties with different scale coverage and described a classical two-genic system, where various combination of four alleles caused scale coverage from fully covered (wild type) to complete scale-loss (nude). Through a forward genetic screen aiming to identify mutations causing phenotypic changes in adult zebrafish, the German team has identified a mutation in the fgfr1a locus that resulted in the loss of most scales in a homozygous situation (spiegeldanio). When individuals of common carp possessing a very similar phenotype (mirror) were genotyped, one of the fgfr1 paralogs, fgfr1a1, was found to carry a mutation. Mammals have a single fgfr1 gene, the function of which is essential during early development. Apparently, the presence of multiple fgfr1 paralogs in teleosts (two in zebrafish and four in carp) allows for function sharing during early development, thus allowing for the study of mutations causing late phenotypes.
Currently, we are searching for the second gene, the mutation of which is causing early lethality in homozygous form and complete loss of scales in heterozygous form (together with homozygous mutant fgfr1) in common carp. We have collected individuals carrying the four principal scale pattern types (fully scaled, linear, mirror and nude) from both subspecies of common carp. We are following a map-based cloning approach and a candidate gene approach in parallel in order to identify and functionally characterize the second gene involved in the process.