17:15 〜 18:30
[BPT02-P02] 浮遊性有孔虫における無性生殖の発見とその意義
キーワード：浮遊性有孔虫、無性生殖、共生藻伝播、生活史
Marine protists have developed diverse modes of reproduction such as binary fission, multiple fission, and budding as means of asexual reproduction, and gametogamy and gamontogamy as means of sexual reproduction. Foraminifera, a group of protists in the Rhizaria, comprise mainly benthic species that generally reproduce both sexually and asexually alternately in their life cycles. However, for planktonic species, gamete release alone has been frequently observed in laboratory cultures. It has been taken as evidence that these organisms produce new generations exclusively by sexual reproduction. We report here the first observation of asexual reproduction in Globigerinita uvula, based on the evidence of the vertical transmission of their endosymbionts.
The foraminiferal sample was collected during Cruise MR13-04 of the R/V Mirai in the western North Pacific, July 2013. Four days after isolation, we found a matured individual releasing numerous two-chambered tests from its aperture. The juvenile specimens were immediately separated and utilized for either (1) DNA analysis, (2) culturing and observation, or (3) active fluorometric measurement to investigate photosymbionts. Based on the 18S rDNA analysis, the juveniles were identified to be G. uvula. Based on the morphological features, the reproduced matured individual was identified to be G. uvula as well, which convinced us that it is the parent of the juveniles. Chlorophyll active fluorescence was detected from the offspring through the culture period, and the intracellular algae were identified to be a pelagophyte alga phylogenetically closest to Pelagomonas calceolata. The presence of symbionts in the newborn, two-chambered offspring indicates that the symbionts were probably transferred vertically from the parent. The vertical transmission of symbionts cannot occur via sexual reproduction since the gametes are too small to hold symbionts. Therefore, our observation strongly suggests that the offspring were asexually reproduced by multiple fissions together with the symbionts.
The trigger for asexual reproduction and its ecological or evolutionary roles remain unknown. Considering that G. uvula is known to dominate opportunistically at times of high phytoplankton production, their ability of asexual reproduction may support their rapid population growth under changing environment.
The foraminiferal sample was collected during Cruise MR13-04 of the R/V Mirai in the western North Pacific, July 2013. Four days after isolation, we found a matured individual releasing numerous two-chambered tests from its aperture. The juvenile specimens were immediately separated and utilized for either (1) DNA analysis, (2) culturing and observation, or (3) active fluorometric measurement to investigate photosymbionts. Based on the 18S rDNA analysis, the juveniles were identified to be G. uvula. Based on the morphological features, the reproduced matured individual was identified to be G. uvula as well, which convinced us that it is the parent of the juveniles. Chlorophyll active fluorescence was detected from the offspring through the culture period, and the intracellular algae were identified to be a pelagophyte alga phylogenetically closest to Pelagomonas calceolata. The presence of symbionts in the newborn, two-chambered offspring indicates that the symbionts were probably transferred vertically from the parent. The vertical transmission of symbionts cannot occur via sexual reproduction since the gametes are too small to hold symbionts. Therefore, our observation strongly suggests that the offspring were asexually reproduced by multiple fissions together with the symbionts.
The trigger for asexual reproduction and its ecological or evolutionary roles remain unknown. Considering that G. uvula is known to dominate opportunistically at times of high phytoplankton production, their ability of asexual reproduction may support their rapid population growth under changing environment.