Conservation paleobiology on Minami-Daito Island, Okinawa, Japan: anthropogenic extinction of cave-dwelling bats on a tropical oceanic island.

BACKGROUND: With strong environmental and geographic filtration, vertebrates incapable of flying and swimming are often extirpated from island ecosystems. Minami-Daito Island is an oceanic island in Okinawa, Japan that harbors the Daito flying fox (Pteropus dasymallus daitoensis), a subspecies of the fruit bat and the only extant mammal endemic to the island. However, the skeleton of a cave-dwelling bat Rhinolophus sp. and fossil guano were briefly reported in a previous study. METHODS: Here, we present evidence for the anthropogenic extirpation of two species of cave-dwelling bats (Miniopterus sp. & Rhinolophus sp.) from Minami-Daito Island. Our goal is to reliably constrain the ages of the extirpated bat species by a multiproxy approach. Because skeletal materials did not preserve sufficient bone collagen for direct radiocarbon dating, we alternatively examined guano-like deposits based on SEM observation and Fourier-transform infrared spectroscopy (FTIR) along with stable carbon and nitrogen isotope analyses for possible indirect dating. We also examined stable carbon isotopes in bone apatite, assuming that an isotopic signal of C4 plants on the bat bones links to sugarcane plantation on the island based on the historical knowledge that early human settlers quickly replaced the island's native C3 forests with sugarcane (C4 perennial grass) plantation from 1900 onward. RESULTS: Our cave survey documents the remains of Miniopterus sp. from the island for the first time. Based on the unique taphonomic conditions (unpermineralized bones, disarticulated skeletons closely scattered without sediment cover, various degrees of calcite crystal growth around bones) and a radiocarbon age of a humic sample, we suggest that the maximum age constraint of Miniopterus sp. and Rhinolophus sp. is 4,640 calBP. Based on a series of analyses, we conclude that the guano-like deposits are composed not of bat guano but mainly of humic substances; however, a hydroxyapatite crust associated with bat-lying stalagmites may be derived from bat feces. Stable carbon isotope analysis of bone apatite revealed C4 signals in various degrees, confirming that small populations of cave-dwelling bats persisted on Minami-Daito Island after 1900. CONCLUSIONS: The results of this study indicate that these populations remained rather small and did not leave many generations and that the estimated ages can be bracketed from 4,640 calBP to the post-1900 (perhaps, until the 1950s). They likely faced a continuously high mortality risk due to severe anthropogenic stresses on the island, where most of the forests were turned into sugarcane plantations within a few decades in the early 20th century. A result of hearing surveys to local residents suggests the latest remnants most likely disappeared on the island concurrently with the introduction of chemical pesticides after World War II.

Biosynthesis of hibarimicins. I. 13C-labeling experiments.

Biosynthesis of hibarimicin was studied based on the feeding experiments with 13C labeled acetates. All carbons in the aglycon, except for the methoxy carbons, were derived from acetate. The carbon framework of the aglycon was proved to be constructed by dimerization of an intermediate which was biosynthesized via the decarboxylation and skeltal rearrangement starting from an undecaketide. The rearrangement was confirmed by detecting the long range (three-bond) coupling between two carbons in the difference spectra of selective 13C decoupled INADEQUATE of hibarimicin B labeled with sodium [1,2-13C2] acetate.

Biosynthesis of hibarimicins. II. Elucidation of biosynthetic pathway by cosynthesis using blocked mutants.

The biosynthetic pathway of hibarimicin (HBM) was proposed on the basis of the experimental results obtained by using blocked mutants of Microbispora rosea subsp. hibaria TP-A0121, the HBM producer. In its biosynthesis, the oxidative coupling of the aromatic undecaketide unit generates a symmetrical aglycon HMP-Y1 (hibarimicin-mutant product Y1), which is oxidatively modified to hibarimicinone, the HBM aglycon. The following glycosylation of hibarimicinone gives rise to the HBM complex. We identified that HMP-Y1 prepared by methanolysis of HMP-Y6, a glycosylated metabolite from a blocked mutant, was the key intermediate: transformation of 13C-labeled HMP-Y1 to HBM B was confirmed by NMR measurements. Mutant strain produced another type of aglycon HMP-P1 in which the coupled polyketide units were intramolecularly bridged by the ether bond. This metabolite also arose by the spontaneous elimination of methanol molecule from hibarimicinone.

Biosynthesis of hibarimicins. III. Structures of new hibarimicin-related metabolites produced by blocked mutants.

Structures of metabolites produced by blocked mutants of Microbispora rosea subsp. hibaria TP-A0121, hibarimicin-producer, were determined by spectroscopic analysis. HMP-Y6 is the dimer of the west half of hibarimicin B, the aglycon of which is the genuine biosynthetic intermediate. HMP-P1 is the shunt product arising from the release of a methanol molecule from hibarimicinone. HMP-P4, the glycoside of HMP-P1, is glycosylated with two amicetoses and two digitoxoses same as hibarimicin B. HMP-M1, M2, M3 and M4 are shunt products derived from the monomeric undecaketide intermediates.