Phylogenetics Of Cucumis Based On Michondrial Sequences And Inheritance Features

Cucumber (Cucumis sativus L.) and melon (C. melo) are Cucurbitaceae Cucumis vegetable crops. Correct phylogenetic relationships of genera Cucumis are both challenging and necessary for breeding of melon and cucumber and ongoing efforts were made to clarify their relationship but obtained contrasting findings. Mitochondria in Cucumis possess enormous huge genome sizes and high frequency of intermolecular recombination, which may offer much information for the evolutionary study. However, mitochondria DNA (mtDNA) in Cucumis as a source of phylogenetic polymorphism had been neglected in the past. Although plants generally show maternal transmission of the organellar genomes, but the mitochondrial genome of cucumber is paternally transmitted and the plastid DNA is maternally inherited, and which had not been certified by cytological observation.To test the phylogenetic relationship of melon and cucumber based on mtDNA,35 pairs of mtDNA consensus primers were employed for DNA fragment length variation and 2 amplified fragments were sequenced from 22 accessions in Cucurbitaceae for sequence variation analysis. Fragment length analysis revealed the existence of two groups (one group composed of C. melo, C. sativus, C. anguria, and C. spp; the other contained the others in African wild species), while sequence variation analysis supported into three groups, C. melo. var. melo and C. melo. var. conomon were grouped as CM, which showed considerable genetic affinities with the one consisting C. sativus. var. sativus, C. sativus. var. hardwickii and C. hystrix, and all the wild African species were grouped together. Moreover, comparative analysis showed that the relationships presented here agree with the trees constructed with concatenated multigene alignment of the chloroplast DNA (cpDNA, rbcL and rbl20-rps12 space region) and internal transcribed spacer (ITS) datasets. The information on novel Cucumis germplasms provided here reveals the closer relationship between cucumber and melon than that between wild African relatives and melon.Then we explored the DAPI-MTG double staining system used in Cucumis and then observed the mitochondrial distribution of adherent and mature pollens in Cucumis. The results showed that pollen morphology including surface texture and villus between different variants in a species was nearly the same, but different from the other genus, which revalidate the importance of the structural characteristics of pollen for species identification. More importantly the observations here also indicated that all the species in Cucumis display potential biparental or paternal mitochondrial inheritance. Meanwhile, although mitochondrial DNA were observed in the adherent pollen of the genara exhibit maternal cytoplastmic inheritance such as Citrullus lanatus but no organelle DNA signals were detected in mature male reproductive cells, revealing mitochondrial DNA digestion have happened during this process. The mechanism of how the mitochondria DNA persist or even amplified in pollen of genera Cucumis need to be explored in the future.