Compared to their mainland relatives, Channel Islands foxes are dwarves with little genetic variation. In fact, one of these island populations may be the least genetically variable population of all wild animals, according to findings published in Current Biology last week.
Channel Island foxes (Urocyon littoralis) evolved from gray foxes brought over to the islands by humans some 7,000 years ago. Nowadays, they live on six of California’s Channel Islands. They’re about two-thirds the size of foxes living on mainland southern California, and like many island species, they’re not really afraid of humans. These isolated island populations have persisted for thousands of years at extremely small population sizes. But in the last couple of decades, four of these subspecies – from San Miguel, Santa Rosa, Santa Cruz, and Santa Catalina – were listed as endangered after predation from non-native golden eagles and the introduced canine distemper virus caused catastrophic declines.
A team led by UCLA’s Robert Wayne sequenced the genomes of Channel Island foxes representing all six populations as well as one mainland gray fox (Urocyon cinereoargenteus) from southern California using DNA samples collected in 1988 before their recent declines, with the plan to form a model for long-term conservation of small endangered populations.
Not only do island foxes have drastically decreased genetic variation, they also have high levels of deleterious mutations compared to the gray fox. That suggests selection hasn’t purged their genome of harmful variants. Specifically, island foxes show a 3- to 84-fold reduction in what’s called heterozygosity. That’s when an individual inherits different variants of a gene from its parents.
In particular, the San Nicolas Island fox showed the most extreme reduction of heterozygosity: They have a near absence of variation. After sequencing the genomes of two San Nicolas individuals, the team found that they were nearly identical. This unique instance of "genomic flatlining" likely shouldn't bode well, yet this small population has managed to survive.
"The degree to which the San Nicolas foxes have lost genetic variation is remarkable, upholding a previous observation that they may be the least genetically variable population of wild animals known," Wayne said in a statement. "It suggests that under some conditions, genetic variation is not absolutely essential for the persistence of endangered populations."