Nile crocodile (Crocodylus niloticus) genetic diversity and population structure, within the lower Kunene and Okavango Rivers of northern Namibia

Versfeld, William F. (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The Nile crocodile has experienced numerous stages of illegal hunting pressures in the mid-20th-century across most of the species’ distribution. The reduced Nile crocodile populations have shown partial recovery and it is currently considered as a “lower risk” / “least concern” species on the Red List of International Union for Conservation of Nature. In Namibia, however, the Nile crocodile is recognised as a protected game species under the Nature Conservation Ordinance No 4 of 1975, allowing trophy hunting of the species only with the issuing of a hunting licence. Census and genetic data of the Nile crocodile is limited or non-existing in Namibia and the country has recently developed a species management plan to conserve the wild populations. During 2012 an aerial survey was conducted along the Lower Kunene River to estimate the abundance and distribution of the Nile crocodile population, by the use of a recently developed N-mixture model. Within the Lower Kunene River system a direct count revealed 562 crocodiles regardless of size, and an estimated population size of 806 individuals, after bias correction. The analyses suggested the class-structured model produced unbiased estimates of the Nile crocodile population in the Lower Kunene River system. To contribute to the conservation efforts of the Nile crocodile in the Lower Kunene River, the study also assessed the genetic diversity and structure within the Kunene and Okavango River system in comparison to neighbouring river basins. This study aimed to develop molecular markers, to assess the patterns of genetic diversity and population structure generated from 11 Short Tandem Repeats and the mitochondrial DNA, control region. The Lower Kunene and Okavango populations indicated a recent divergence with a single haplotype shared among the 64 samples sequenced and interestingly the haplotype was shared with populations in Gabon and Uganda. Moreover, there was no sharing of haplotypes found between the Lower Kunene and Okavango and the Lower Shire River system. Estimated for pairwise population differentiation, F-statistics, AMOVA and factorial correspondence analysis (FCA), based on Short Tandem Repeats, indicated significant structuring among the populations. Additionally, Bayesian clustering analyses detected three putative ancestral gene pools, of which two were present in the Okavango River population, supporting the findings of the Nile crocodile to be structure according to river basin formation. Despite no expansion or population bottleneck detected in the Nile crocodile populations, a contemporary genetic bottleneck may have gone undetected due to the crocodile’s long-life span and breeding between overlapping generations. The contemporary restriction of gene flow and historical river topography are the most likely cause of genetic structure in the Nile crocodile populations of today. Even though the Kunene and Okavango Nile crocodile populations are experiencing different environmental and evolutionary pressures, the genetic data suggest a single evolutionary significant unit, with two management units. The Okavango River had a broad sampling range for the study, however both river populations will require more samples to validate fine-scale genetic structure.

AFRIKAANSE OPSOMMING: Die Nyl krokodil het talle fases van onwettige jag ervaar in die middel van die 20ste eeu oor meeste van die spesie se verspreidingsgebied. Die verminderde Nyl krokodil bevolking het gedeeltelike herstel en word tans beskou as 'n laer risiko / minste kommer spesie op die rooi lys van die Internasionale Unie vir die Bewaring van die Natuur. In Namibië, is die Nyl krokodil erken as 'n beskermde wildsoort onder die Natuurbewaring Ordonnansie Nr. 4 van 1975, sodat trofeejag van die spesie net met die uitreiking van 'n jaglisensie uitgevoer mag word. Sensus en genetiese data vir die Nyl krokodil is beperk of nie-bestaande in Namibië en die land het onlangs 'n spesie bestuursplan, om die wilde bevolkings bewaar, ontwikkel. Gedurende 2012 is 'n lug telling opname langs die ‘Lower’ Kunene Rivier gedoen om die digtheid en verspreiding van die Nyl krokodil bevolking te skat, deur die gebruik van 'n onlangs ontwikkelde ‘N-mixture’ model. Die ‘Lower’ Kunene Rivier direkte telling, skat ongeveer 562 krokodille ongeag die grootte, en 'n geskatte bevolkingsgrootte van 806 individue na sydigheidskorreksie. Die analise blyk dat die klas gestruktureerde model kan onbevooroordeelde ramings van die Nyl krokodil bevolking in die ‘Lower’ Kunene Rivier stelsel produseer. Om verder by te dra tot die bewaringspogings van die Nyl krokodil in die ‘Lower’ Kunene Rivier, poog die studie om die genetiese diversiteit en -struktuur binne die Kunene- en Okavango Rivier stelsel, in vergelyking met die naburige rivierbekkens, te ondersoek . Elf mikrosatelliet merkers en mitochondriale DNA is vir die doel aangewend. Die ‘Lower’ Kunene en Okavango bevolkings dui op 'n onlangse divergensie met 'n enkele haplotipe wat gedeel word tussen die 64 monsters. Die haplotipe word interessantlik gedeel met bevolkings in Gabon en Uganda. Verder was daar twee takke van haplotiepes gevind tussen die ‘Lower’ Kunene en Okavango en die ‘Lower’ Shire-rivier stelsel. Na raming van paarsgewyse bevolking differensiasie, F-statistieke, AMOVA en faktoriaal korrespondensie analise (‘FCA’), gebaseer op mikrosatelliete, is daar beduidende genetiese strukturering tussen verskeie bevolkings. Daarbenewens, bevind ‘Bayesian’ groeperingsanalise dat daar drie vermeende voorvaderlike geen poele bestaan, waarvan twee teenwoordig was in die Okavango Rivier bevolking. Hierdie data ondersteun vorige waarneming dat Nyl krokodil populasiestruktuur beïnvloed word deur rivier vorming. Ten spyte van die beraamde afwesigheid van bevolkingsuitbreiding of bevolkingsbottelnekke in die Nyl krokodil, kan 'n kontemporêre genetiese bottelnek ongemerk gegaan het as gevolg van die krokodil se lang lewensduur en teling in oorvleueling generasies. Die kontemporêre beperking van genevloei en historiese rivier topografie is die mees waarskynlike oorsaak van die beskryfde genetiese struktuur in die Nyl krokodil bevolking. Selfs ervaar die Kunene en Okavango Nyl krokodil bevolkings verskillende omgewings- en evolusionêre druk, is daar slegs ʼn enkele evolusionêre beduidende eenheid, met moontlik twee bestuurseenhede. Die Okavango Rivier het 'n breë monsterneming reeks vir die studie; Maar beide rivier bevolkings sal meer monsters benodig om fyn skaal genetiese struktuur te ondersoek.

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