A diffuse glow, through the ages

Gary Boas

Alhough bioluminescence in fire-flies has been studied extensively, its anatomical and biochemical origins still are relatively unknown. The organs that generate the luminescence can be found in various locations along the body and can exhibit varying degrees of complexity, preventing researchers from applying findings from one family to another. To date, detailed studies of such origins have been performed primarily for Photuris and Photinus fireflies.

Recently, investigators at Universidade Federal de São Carlos in Sorocaba, Brazil, reported that larvae of the Brazilian Aspisoma lineatum firefly can emit a continuous weak, diffuse glow throughout the body — detectable only by the dark-adapted eye.

“The emission of low-level bioluminescence prompted us to investigate the histological origin,” said Vadim R. Viviani, one of the study’s authors, to identify the tissues in which the glow originates. Thus, they explored the anatomical and histological origins of this phenomenon.

Researchers probed the anatomical and histological origins of a weak, diffuse glow throughout the body of the Brazilian A. lineatum firefly, using a sensitive CCD camera system. These images show mature larva, dorsal (A); mature larva, ventral (B); mature larva, lateral (C); larva after ecdysis (D); prepupa (E); pupa, ventral (F); late pupa, ventral (G); and late pupa, dorsal (H). Reprinted with permission from Photochemical & Photobiological Sciences.

The researchers studied this bioluminescence thanks to a sensitive CCD camera system from Atto Corp. of Tokyo. They isolated lanterns and spheric bodies from each larva and measured luciferase activity, in mV, using a luminometer. Finally, they recorded bioluminescence spectra using a Hitachi spectrofluorometer.

The experiments revealed, in addition to the glow provided by the larval fireflies’ lanterns, a continuous basal luminescence throughout their bodies. The investigation also showed that this luminescence arises from the fat body, which consists of small lobes distributed throughout the body cavity, thus offering evidence of photocytes — one of the basic building blocks of the lanterns — originating in fat body cells.

The study also offered some clues as to the evolutionary origins of the lanterns. Researchers believe that the development of lanterns was preceded by a lanternless bioluminescent stage in fireflies. Viviani and colleagues noted that the basal diffuse bioluminescence probed in the present study may be a vestige of this early lanternless stage; the basal bioluminescence in this early stage may have been gradually restricted to smaller and smaller areas until it became the lanterns so common to today’s fireflies.

The research continues. The investigators hope to identify the photogenic cells underlying the basal luminescence found in the A. lineatum firefly.

“This work is still under way through histological and biochemical studies,” Viviani said. The researchers also intend to study the basal luminescence of tissues from other bioluminescent families. For these, they might need to employ a different imaging system. “The CCD camera system [we used for the present study] was sensitive enough,” Viviani explained. “However, more sensitive systems are needed for lower intensities of bioluminescence.”

Photochemical & Photobiological Sciences, April 2008, pp. 448-452.