Molecular Cytogenetics of Reproductive Failure in Gorillas
Maka, a male lowland gorilla, was born at the San Diego Zoo in 1995.
Several aspects of the conservation efforts of zoos through maintaining captive populations of western lowland gorillas invoke novel genetic questions. A large founder size for western lowland gorillas implies that a remarkable amount of genetic variation is represented in captive populations, especially in comparison to most others taxa held in zoos. Depending on the extent to which the founder gorillas sampled the genetic variation across the range of western lowland gorillas, the captive population may reflect the evolutionary patterns of radiation and occupancy of gorilla populations from the time of their divergence with a common ancestor of humans and chimpanzees. Certainly, studies of genetic variation indicate that gorillas exhibit extraordinary genetic variability, especially in comparison with humans. Deep divisions in the mitochondrial phylogeny suggest ancient separation of lineages. These data have been used to infer the existence of refugia during periods when the forests of western and central Africa waned and were reestablished over the last several million years.
The extent of the genetic differentiation of different populations of western lowland gorillas is roughly similar to that between the two species of chimpanzee. Thus, differentiation within western lowland gorillas may have unpredictable impacts when individuals from the distinct genetic lineages interbreed, as has come to be the case in the management of captive western lowland gorillas. An “early warning” of such potential genetic incompatibilities would involve reduced reproduction, the production of abnormal individuals, and fetal wastage. A cytogenetic survey of western lowland gorillas has revealed remarkable amounts of variation in heterochromatin and homogeneous staining regions, extreme variation in Y-chromosome length and other features that have received only little attention and much of this decades ago.
In 1995, a male western lowland gorilla, Maka, was born at the San Diego Zoo and later observed to exhibit growth retardation. Cytogenetic analysis revealed a remarkable deletion of the terminal portion of the long arm of the third chromosome. The possibility exists that this chromosomal abnormality was generated because of structural factors involving dissimilarity of his parents’ chromosomes through an event (paracentric inversion) that can only be detected by applying advanced methods of molecular genetic analysis, including FISH (fluorescence in situ hybridization).
In 2006, a trisomic fetus was aborted in Seattle (possibly trisomy 15). The production of genetically abnormal individuals and the impacts that they have on gene pool conservation and captive space and husbandry efforts can be significant. This study is undertaking detailed molecular cytogenetic studies to evaluate the possibility of chromosomal risk factors leading to genetic incompatibilities within the captive population of western lowland gorillas and the production of compromised offspring.
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