Non-concerted ITS evolution in Mammillaria (Cactaceae)

Molecular studies of 21 species of the large Cactaceae genus Mammillaria representing a variety of intrageneric taxonomic levels revealed a high degree of intra-individual polymorphism of the internal transcribed spacer region (ITS1, 5.8S rDNA, ITS2). Only a few of these ITS copies belong to apparently functional genes, whereas most are probably non-functional (pseudogenes). As a multiple gene family, the ITS region is subjected to concerted evolution. However, the high degree of intra-individual polymorphism of up to 36% in ITS1 and up to 35% in ITS2 suggests a non-concerted evolution of these loci in Mammillaria. Conserved angiosperm motifs of ITS1 and ITS2 were compared between genomic and cDNA ITS clones of Mammillaria. Some of these motifs (e.g., ITS1 motif 1, 'TGGT' within ITS2) in combination with the determination of GC-content, length comparisons of the spacers and ITS2 secondary structure (helices II and III) are helpful in the identification of pseudogene rDNA regions.  

鲽科10种鱼类核糖体ITS2序列比较分析

为了解鲽科鱼类ITS2序列的多态性特征，本研究获得了鲽科10种鱼类310条ITS2序列，长度在419 ~ 486 bp之间。种内序列长度差异最小的为粒鲽（Clidoderma asperrima）（419 ~ 420 bp）和太平洋拟庸鲽（Hippoglossoides elassodon）（419 ~ 420 bp），其次为北岩鲽（Lepidopsetta polyxystra）（447 ~ 452 bp）和刺黄盖鲽（Limanda aspera）（457 ~ 463 bp），松木高眼鲽（Cleisthenes pinetorum）（452 ~ 462 bp）和圆斑星鲽（Verasper variegatus）（465 ~ 479 bp）的种内序列差异分别为10 bp和14 bp；其余4种鱼类根据长度差异（14 ~ 32 bp）分型为长（A型）、短（B型）序列类型，同时检测存在重组类型（R型），其中长度差异最大的是钝吻黄盖鲽（Pseudopleuronectes yokohamae）（454 ~ 486 bp），其次为尖吻黄盖鲽（P. herzensteini）（433 ~ 458 bp）、虫鲽（Eopsetta grigorjewi）（420 ~ 439 bp）、星突江鲽（Platichthys stellatus）（466 ~ 480 bp）。通过双参数模型（K2P）计算遗传距离可见，种内遗传距离多集中于0.002 ~ 0.027之间，仅星突江鲽和尖吻黄盖鲽因类型差异导致较高数值（0.043和0.053）；不同物种间遗传距离在0.046 ~ 0.180之间。10种鱼类ITS2的GC含量为63.95% ~ 70.16%；9种鱼类的二级结构均为具有5个分支（HelixⅠ ~ Ⅴ）的闭合环状结构，仅圆斑星鲽中由于存在Helix Ⅳ变异形成Helix Ⅳ-a和Helix Ⅳ-b而具有6个分支。基于ITS2构建的鲽科10种鱼类的系统进化树显示，不同种鱼类的克隆序列均单独聚支。序列的多态性特征分析表明，在具有不同序列类型的虫鲽、星突江鲽、尖吻黄盖鲽和钝吻黄盖鲽4种鲽科鱼类中，ITS2以非协同进化的方式存在，而其他6种鱼类为协同进化；虽然存在种间K2P遗传距离小于种内的个例，但ITS2在属间不同物种的区分上具有适用性。本研究结果丰富了鲽形目鱼类的ITS2数据，也将为鱼类的核糖体RNA序列多态性的研究提供科学依据。  

Phylogenetic relationships of Mangifera species revealed by ITS sequences of nuclear ribosomal DNA and a possibility of their hybrid origin

 The phylogenetic relationships among 14 Mangifera L. species of Thailand were analyzed by comparing sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA (nrDNA). Parsimony and neighbor joining (NJ) analyses revealed that the common mango (M. indica L.) was closely related to M. laurina Bl., M. sylvatica Roxb., and M. oblongifolia Hook. f. Mangifera foetida Lour. and M. odorata Griff. were also related to M. indica in both parsimonious and NJ trees, although these two species are classified into a different subgenus (subgenus Limus) from the subgenus Mangifera to which M. indica belongs. ITS sequence analysis revealed that several species have nucleotide additivity (two different nucleotides at the same locus) at several sites in the ITS region. Also, M. indica had several polymorphisms among cultivars. This finding may suggest a possibility of hybrid origin of Mangifera species, although Mangifera species are all assumed to be diploid having chromosome number of 2n=2x=40. Received February 7, 2001 Accepted October 28, 2001