CHS基因起源初探及其在被子植物中的进化分析

利用PCR与TAIL-PCR方法,从半月苔(Lunulariacruciata(L.)Dum.exLindb.)中获得了一段长约1000bp的基因片段,它与已知的CHS基因在核苷酸水平上的相似性大于56%,在氨基酸水平上的相似性大于60%,所推断的氨基酸序列中酶反应的4个催化位点与已知晶体结构的紫花苜蓿MCHS2A上的催化位点相同,首次证明了苔类植物中可能存在类CHS基因,将CHS基因的起源时间推到苔藓类植物出现之前。以该序列和两种蕨类植物(Psilotumnudum(L.)Griseb.和EquisetumarvenseL.)的CHS序列作为外类群,应用邻接法、最大简约法和最大似然法分别构建了被子植物的CHS的分子系统树。结果表明,大部分科中的CHS分布在不同的分支上,而十字花科、豆科和禾本科各自聚成一个单系类群。以邻接树为依据,对茄科、旋花科和菊科的CHS基因进行了相对碱基替换速率的检测,发现这三个科内或科间序列的替换速率不一致。被子植物的CHS基因在基因拷贝数目、碱基替换速率以及重复/丢失事件的发生上都存在较大的差异,这种差异可能与被子植物的生活史、生活环境、花的特性以及对外界的防御系统等的多样性相关。

A Preliminary Study on the Origin and Evolution of Chalcone Synthase (CHS) Gene in Angiosperms

By using Thermal Asymmetric Interlaced PCR (TAIL-PCR) method, a DNA fragment of about1 000 bp was amplified and cloned from a liverwort species (Lunularia cruciata (L.) Dum. ex Lindb). Thenucleotide sequence of this fragment and its deduced amino acid sequence shared about 56% and 60%identity with those of exon 2 of CHS genes from vascular plants respectively. The four characteristiccatalyzing sites of CHS were found conserved in the deduced amino acid sequences of the fragment whencompared with other CHS sequences. This is the first report of cloning a CHS-like gene from liverworts,suggesting that the origin of CHS genes may predate liverworts. Using the CHS-like sequence from L.cruciata and CHS sequences from two fern-alien species, Psilotum nudum (L.) Griseb. and Equisetumarvense L., as outgroups, the phylogenetic trees of about 250 CHSs from 29 families of angiosperm plantswere constructed by using the neighbour-joining (NJ), maximum parsimony (MP) and quartet puzzle (QP)methods. The results showed that the CHSs from most plant families were separated into two or moreclades while sequences from the families Brassicaceae, Fabaceae and Poaceae were each grouped into anindependent monophyletic clade. The relative base substitution rates were estimated for CHS genes inthree plant families, Solanaceae, Convolvulaceae, and Asteraceae, where the heterogeneity rate wasdetected both within and among the families. Results indicated that CHS genes in angiosperm plants weregreatly diverse in terms of copy number, base substitution rate, and duplication/deletion events, whichmight be correlated with the diversity of life history, habitat, floral characters, and defense system ofangiosperm plants.