3. Recent Genetic Research

white tiger genetic studies

In recent years there have been many studies and breakthroughs in genetic research. A few of these are specific to the White Tiger, while some are about the tiger in general, but have implications for white tigers too.

In summary:

  • Tigers in the wild have fragmented and low overall genetic diversity (which is dangerous for species survival), so every bit of genetic variation – in captivity or the wild – is valuable to preserve.
  • Captive tigers, and white tigers specifically, have been found to hold genetic variation no longer found in the wild, and therefore are valuable.
  • Cross-bred tigers are also a valuable source of genetic diversity and it may even be necessary for the tiger’s survival to cross-breed subspecies.
  • The gene that produces the white coat has been identified, and found to affect pigmentation only – it does not cause defects.
  • White tigers are not all inbred

Following are 6 of the important recent studies ordered by year of publication. (Links to the original study or article are in green).


[1] 2008: Captive Tigers Hold Valuable Genetic Diversity

The Study: Subspecies Genetic Assignments of Worldwide Captive Tigers Increase Conservation Value of Captive Populations

Luo, Shu-Jin et al. Current Biology , Volume 18 , Issue 8 , 592 – 596

Scientists developed a method for testing the subspecies genetic ancestry of tigers and found that 49 of their test group of 105 captive tigers were a “pure bred” subspecies. This included 7 out of 50 tigers that were previously of unknown origin. This means that there could be a large number of “extra” pure-bred tigers available for subspecies breeding programs (14-23%). They also found that:

“The tested captive tigers retain appreciable genomic diversity unobserved in their wild counterparts”

“A wide-ranging identification of captive VSA tigers to assess their potential for inclusion into comprehensive, integrated in situ and ex situ management plans could significantly increase population sizes and help maintain genetic variability and population viability of this iconoclastic species.”

“Currently admixed tigers are removed from managed-breeding programs when they are identified. However, in the future consideration also may be given to certain hybrid tigers that carry genotypes of value for conservation purposes.”

 


[2] 2009: White Tigers Hold Genetic Diversity no longer in the wild

The Study: mtDNA Indicates Profound Population Structure in Indian Tiger (Panthera tigris tigris)

Sharma et al; Conserv Genet (2009) 10:909–914

Scientists analyzed mitochondrial DNA (maternally inherited) from 13 Indian wild populations of the Bengal tiger, plus captive zoo tigers.

They identified haplotypes (DNA variants) that occurred either throughout all populations, or only in some. One of these haplotypes was found only in captive white tigers. (Note that this is not referring to the white gene itself, which was identified later, but to other DNA variations in this lineage). They also noted that the Bengal tiger has low diversity overall.

“10 Indian tiger haplotypes are now known, which differ from one another by only a few mutations. This low diversity is typical also for other tiger subspecies”

“In our study, haplotype TIG* 9 was only found in white tiger specimens from Zoo Delhi representing the extinct population from Rewa (Central India). Possibly, animals carrying this haplotype were taken to captivity before this mitochondrial lineage became rare or even extinct in the wild.”

 


[3] 2013: Wild Tigers Have Lost 93% of their Historic Genetic Diversity

Article: Indian Tigers’ Genetic Diversity: Population Faces Extinction For Lack Of DNA Variety, Study Says “93 percent of the tiger DNA variants from that historical period are no longer present in the current tiger population.”

Sara Gates, The Huffington Post, 05/15/2013

 

“A new study from Cardiff University has researchers concerned that Indian tigers face extinction because of a lack of “genetic diversity.”Researchers compared current DNA samples from tigers on the Indian subcontinent to genetic data obtained during the time of the British Raj, a period of British rule in India from 1858 to 1947. They found that 93 percent of the tiger DNA variants from that historical period are no longer present in the current tiger population….. The lack of genetic diversity in Indian tigers presents an obvious “red flag” for conservationists…”

The Study: Demographic Loss, Genetic Structure and the Conservation Implications for Indian Tigers 
Samrat Mondol, Michael W. Bruford, Uma Ramakrishnan. Proc. R. Soc. B 2013 280 Published 15 May 2013

“Genetic variation can be crucial for species survival and adaptation to environmental change”

Scientists were concerned that an increase in numbers for the tiger population in India was not a good enough indication of subspecies recovery, because:

“Loss of genetic variation, while rarely posing the immediate threats associated with demographic loss and environmental stochasticity, can nevertheless ultimately compromise populations and may lead to extinction”

They sampled DNA from museum specimens of tigers, from the period of about 1850-1950. They compared this with DNA from modern wild tiger populations. They found that:

“… a large number of haplotypes present historically in India have been lost. Apart from a decrease in genetic variation per se, only 7 per cent of the historical mitochondrial haplotypes are represented in the contemporary samples”

They also found increased population structure in the Indian tiger (ie differentiation between isolated populations):

“increased population structure correlates with extinction, suggesting that the trends we observe represent a conservation ‘red flag’…..Current conservation initiatives discuss maintaining connectivity at a landscape level. However, maintaining genetic variation will require population connectivity at much larger spatial scales.”

 


[4] 2013: Crossbreeding Subspecies May be Necessary to Save the Tiger

The Study: A Call for Tiger Management Using “Reserves” of Genetic Diversity

Rachael A. Bay, Uma Ramakrishnan and Elizabeth A. Hadly; Jnl of Heredity, Dec 2013

The Global Tiger Recovery Program aims to double the number of wild tigers by 2022. However, genetic diversity is also extremely important to tiger conservation, so scientists used a predictive genetic model to predict the population growth needed to maintain current genetic diversity of the tiger over the next 150 years. They found that it would need to grow to 60,000 to 98-000 ! Such a large  growth is unrealistic, so they recommended that gene-flow between subspecies, and even using captive tigers, would be necessary to ensure the survival of the tiger as a species.

“The necessity of a large population to house genetic diversity for tigers means that in the absence of gene flow, an unrealistic increase in population size would be necessary. …Thus, managing for the genetic diversity of the species should be prioritized over the riskier preservation of distinct subspecies.”

Article: Increase Genetic Diversity to Save Tigers [The Times of India}

“The results showed that for tiger populations to maintain their current genetic diversity 150 years from now, the tiger population would have to expand to about 98,000 individuals if gene flow across species were delayed 25 years. By comparison, the population would need to grow to about 60,000 if gene flow were achieved immediately. Neither of these numbers is realistic, considering the limited size of protected tiger habitat and availability of prey, among other factors, according to the researchers.”

 

“Because migration and interbreeding among subspecies appear to be “much more important” for maintaining genetic diversity than increasing population numbers, the researchers recommend focusing conservation efforts on creating ways for tigers to travel longer distances, such as wildlife corridors, and potentially crossbreeding wild and captive tiger subspecies.

This is very much counter to the ideas that many managers and countries have now – that tigers in zoos are almost useless and that interbreeding tigers from multiple countries is akin to genetic pollution,” said Hadly. “In this case, survival of the species matters more than does survival of the exclusive traits of individual populations,” says the report.”

 

Article: Diverse gene pool critical for tigers’ survival, say Stanford scholars

Rob Jordan, Stanford Report, April 15 2014

“Conservation plans that focus only on increasing numbers and preserving distinct subspecies ignore genetic diversity, according to the study. In fact, under that approach, the tiger could vanish entirely.”

[5] 2013: The White Gene is Identified

The Study:The Genetic Basis of White Tigers …(or PDF here)

Luo et al, Current Biology June 2013

Scientists identify the gene responsible for the white coat/blue eyes: a change in a single amino acid (A477V) in one pigment-related gene, SLC45A2, that blocks the production of red/yellow pheomelanin while still allowing the black/brown melanin to be produced. This is not at the albino locus, as previously thought, but is  the same gene that causes color variation in humans, horses, and chickens. The scientists concluded that it affected only pigmentation, and is a naturally occurring variant.

Article: White Bengal Tiger enigma solved (R. Prasad, The Hindu, May 24 2013)

“They dismiss the notion that the white tiger trait is a genetic deformity. That matured white tiger adults have been sighted in the wild negates this notion. Despite its low frequency, they emphasise that the mutation is a naturally occurring one and should be considered as a “part of genetic diversity of tigers that is worth conserving.”

Related Article: Save the White Tigers By Shu-Jin Luo and Xiao Xu; Scientific American, October 16, 2014

The truth is that white tigers are the product of a rare but naturally occurring genetic variant within the wild Bengal population. Even so, the experts’ confusion about the subject has been understandable, given the previous lack of precise information on the white tiger’s genetic roots. It was only last year that our team published the work cracking the mystery at last.”

 

[6] 2013: White Tigers are not all Inbred

The Study: Genetic Diversity of White Tigers and Genetic Factors Related to Coat ColorMay 2013, SE Carney 

This study measured the amount of heterozygosity (ie genetic variation) in white tigers and orange tigers, and found that both groups had about the same degree of variation.

“It is apparent that among the white tigers and orange tigers sampled, there is no statistically significant difference in heterozygosity. This indicates that the white tigers included in this study were likely outbred to orange tigers, maintaining higher heterozygosity. Though it is known that early captive white tiger populations originated through inbreeding, it is clear from our results that not all white tigers presently in captivity are significantly inbred.”

 

 

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