We surveyed sequence variation at 5 loci, 9-cis-epoxycarotenoid dioxygenase locus (NCED), ammonium transporter locus (AMT), calmodulin locus (Cal), aquaporin locus (AQU), and Cryj2 locus, using the same 48 samples in a conifer, Cryptomeria japonica. The average nucleotide diversity, pi, at silent sites is 0.0044. The average value of r/u, where r and u are the recombination and mutation rates, respectively, per base pair per generation, is estimated to be about 8.0 using the method of Kuhner et al. (2000). Tests of neutrality, such as Tajima's D, Fu & Li's F*, D* and other statistics, did not give any significant results. Although the frequency spectrum of the nucleotide polymorphism at the 7 loci reported previously suggested some past population structure, the significance disappears after the addition of the data at the 5 loci. At the NCED locus, McDonald-Kreitman test reveals an excess of replacement polymorphisms. Given that there are constraints on replacement polymorphisms in the later half of the gene and thus the gene is not a pseudogene, one explanation for the result would be a relaxation of selective constraints on slightly deleterious variants, probably due to a reduction of population size in C. japonica. At the Cryj2 locus, an excess of amino acid substitutions was found. Cryj2 locus may have experienced adaptive evolution.
我々は、３集団（北陸、関東・東海、岩手）から採取したスギ （Cryptomeria japonica）４８個体を用いて５遺伝子座( NCED、AMT、Cal、 AQU、Cryj2 )の塩基配列多型を調査した。多型のパターンは、任意交配、一定 サイズを仮定したときの予測値と有意差がなかった。しかし、McDonald Kreitman testを行ったところ、NCED遺伝子座で、種内のアミノ酸多型が有意 に多い結果となった。この結果は、弱有害変異の存在を示唆していると考えら れる。また、Cryj2遺伝子座では、種間のアミノ酸置換が有意に多い結果とな り、正の自然選択がこの遺伝子に働いている可能性が示唆された。これらの結 果を、角等(Kado et al 2003)の分子集団遺伝学的研究と、楠見等(Kusumi et al 2002)の分子進化的研究の結果と併せて考えると、スギの進化には弱有害変 異と適応変異がともに重要な役割を果たしていると考えられる。