Janssen Toon, Karssen Gerrit, Topalović Olivera, Coyne Danny, Bert Wim
Nematology Research Unit, Department of Biology, Ghent University, K.L. Ledeganckstraat, Ghent, Belgium.
Center for Medical Genetics, Reproduction and Genetics, Reproduction Genetics and Regenerative Medicine, Vrije Universiteit Brussel, UZ Brussel, Laarbeeklaan, Brussels, Belgium.
PLoS One. 2017 Mar 3;12(3):e0172190. doi: 10.1371/journal.pone.0172190. eCollection 2017.
During sampling of several Coffea arabica plantations in Tanzania severe root galling, caused by a root-knot nematode was observed. From pure cultures, morphology and morphometrics of juveniles and females matched perfectly with Meloidogyne africana, whereas morphology of the males matched identically with those of Meloidogyne decalineata. Based on their Cox1 sequence, however, the recovered juveniles, females and males were confirmed to belong to the same species, creating a taxonomic conundrum. Adding further to this puzzle, re-examination of M. oteifae type material showed insufficient morphological evidence to maintain its status as a separate species. Consequently, M. decalineata and M. oteifae are synonymized with M. africana, which is herewith redescribed based on results of light and scanning electron microscopy, ribosomal and mitochondrial DNA sequences, isozyme electrophoresis, along with bionomic and cytogenetic features. Multi-gene phylogenetic analysis placed M. africana outside of the three major clades, together with M. coffeicola, M. ichinohei and M. camelliae. This phylogenetic position was confirmed by several morphological features, including cellular structure of the spermatheca, egg mass position, perineal pattern and head shape. Moreover, M. africana was found to be a polyphagous species, demonstrating that "early-branching" Meloidogyne spp. are not as oligophagous as had previously been assumed. Cytogenetic information indicates M. africana (2n = 21) and M. ardenensis (2n = 51-54) to be a triploid mitotic parthenogenetic species, revealing at least four independent origins of mitotic parthenogenesis within the genus Meloidogyne. Furthermore, M. mali (n = 12) was found to reproduce by amphimixis, indicating that amphimictic species with a limited number of chromosomes are widespread in the genus, potentially reflecting the ancestral state of the genus. The wide variation in chromosome numbers and associated changes in reproduction modes indicate that cytogenetic evolution played a crucial role in the speciation of root-knot nematodes and plant-parasitic nematodes in general.
在对坦桑尼亚的几个阿拉比卡咖啡种植园进行采样时，观察到由一种根结线虫引起的严重根部瘿瘤现象。从纯培养物中发现，幼虫和雌虫的形态及形态测量数据与非洲根结线虫完全匹配，而雄虫的形态与递减根结线虫的雄虫完全相同。然而，根据它们的细胞色素氧化酶亚基1（Cox1）序列，所回收的幼虫、雌虫和雄虫被确认为属于同一物种，这就产生了一个分类学难题。更添谜团的是，对奥泰法根结线虫模式标本的重新检查显示，维持其作为一个独立物种的地位缺乏足够的形态学证据。因此，递减根结线虫和奥泰法根结线虫被认定为非洲根结线虫的同物异名，在此根据光学显微镜和扫描电子显微镜观察结果、核糖体和线粒体DNA序列、同工酶电泳以及生物学和细胞遗传学特征对非洲根结线虫进行重新描述。多基因系统发育分析表明，非洲根结线虫与咖啡根结线虫、猪屎豆根结线虫和茶根结线虫一起，处于三个主要分支之外。这一系统发育位置通过几个形态学特征得到了证实，包括受精囊的细胞结构、卵块位置、会阴花纹和头部形状。此外，发现非洲根结线虫是一种多食性物种，这表明“早期分支”的根结线虫属物种并不像之前所认为的那样食性狭窄。细胞遗传学信息表明，非洲根结线虫（2n = 21）和阿登根结线虫（2n = 51 - 54）是三倍体有丝分裂孤雌生殖物种，这揭示了根结线虫属内至少有四个独立的有丝分裂孤雌生殖起源。此外，发现马里根结线虫（n = 12）通过两性生殖进行繁殖，这表明染色体数量有限的两性生殖物种在该属中广泛存在，这可能反映了该属的祖先状态。染色体数量的广泛变化以及相关繁殖模式的改变表明，细胞遗传学进化在根结线虫以及一般植物寄生线虫的物种形成过程中起到了关键作用。
