|Budget Amount *help
¥2,000,000 (Direct Cost : ¥2,000,000)
Fiscal Year 1993 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1992 : ¥1,100,000 (Direct Cost : ¥1,100,000)
We have studied biosystematically many and various populations of Closterium ehrenbergii sampled from paddy fields, ponds and lakes in Japan, Taiwan, Nepal and Australia, and recognized so far more than a dozen mating groups (from A to P) primarily based on sezual isolation. Morphological and cytological studies, as well as those on the reproductive barriers of pre- and post-zygotic isolation, clearly suggest that each mating group of this alga be a biological species. In this project, we focused to study two contrasting mating groups of Closterium ehrenbergii. Namely, one is Group A of which natural populations are subjected to the conditions that seasonal and/or occasional occurrences of drought and/or extremely high and low temperatures would select, at least once a hear, against any defective mutation that affects the sezual process of zygote formation and germination. The other is Group P of which natural populations are maintained for many years only by means of vegetative cell divisions without sexual reproduction.
(1) Although both mating types, plus and minus, coexist in single localities in Europe, we have not yet found any place where both the two coexist and all the samples collected so far in Japan contained vegetative cells of only one mating type. Natural populations of only plus were found in north-eastern and central Hokkaido, Akita, Iwate, Yamagata and Fukushima, and those of only minus were found in south-western Hokkaido, Niigata, Nagano, Toyama, Yamanashi and Shizuoka. Based on crossings in almost all possible combinations among isolates from one natural population, we found a dominant or mating-type plus specific, asynaptic mutation.
(2) Although we could not detect such lethal zygotic factors in Group A, we found strong inbreeding depression in selfing and backcrossing F_1 progeny to the same parental strains.