Codon usage bias (CUB) is the nonrandom usage of synonymous codons in which some codons are more preferred to others.CUB can be determined by mutation pressure and selection. Various approaches have been used to understand the pattern of CUB in the mitochondrial ND (MT-ND or ND) genes involved in complex I of respiratory chain in five different classes of Platyhelminthes as no work was reported yet. The present study revealed that the CUB varies across MT-ND genes and the coding sequences showed the richness of A and T. Correspondence analysis implied the effect of mutational pressure and also the pattern of codon usage was different in different classes of platyhelminthes for MT-ND genes. Highly significant correlation was observed between overall nucleotide composition and its 3rd codon position in most of the homogeneous nucleotides such as A% and A3%, T% and T3%, G% and G3%, C% and C3%, GC% and GC3% and also some significant correlations observed among heterogeneous nucleotides in all the five classes for MT-ND genes suggested the role of mutational pressure as well as natural selection in affecting the CUB. Neutrality plot suggested that the contributions of natural selection and mutational pressure varied across different classes of platyhelminthes and also differed in different MT-ND genes.

密码子使用偏倚（CUB）是同义密码子的非随机使用，其中某些密码子比其他密码子更偏爱.CUB可以通过突变压力和选择来确定。由于尚未报道任何工作，已使用多种方法来了解涉及五种不同种类的侧柏的呼吸链复合体I的线粒体ND（MT-ND或ND）基因中CUB的模式。目前的研究表明，MT-ND基因的CUB有所不同，编码序列显示出A和T的丰富性。对应分析暗示了突变压力的影响，并且在不同种类的桔梗中，密码子的使用方式也不同。 ND基因。在大多数同质核苷酸中，如A％和A3％，T％和T3％，G％和G3％，C％和C3％，GC％和GC3，在总核苷酸组成与其第三密码子位置之间观察到高度显着的相关性％，以及在所有五类MT-ND基因的异源核苷酸之间观察到的一些显着相关性表明，突变压力以及自然选择在影响CUB中的作用。中性图表明，自然选择和突变压力的贡献在不同种类的桔梗中有所不同，并且在不同的MT-ND基因中也有所不同。在MT-ND基因的所有五类中，GC％和GC3％以及在所有五类异源核苷酸之间观察到的一些显着相关性表明，突变压力以及自然选择在影响CUB中的作用。中性图表明，自然选择和突变压力的贡献在不同种类的桔梗中有所不同，并且在不同的MT-ND基因中也有所不同。在MT-ND基因的所有五类中，GC％和GC3％以及在所有五类异源核苷酸之间观察到的一些显着相关性表明，突变压力以及自然选择在影响CUB中的作用。中性图表明自然选择和突变压力的贡献在不同种类的桔梗中有所不同，并且在不同的MT-ND基因中也有所不同。