Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, Guiyang, 550025, China.
Institute of Fungus Resources, College of Life Sciences, Guizhou University, Guiyang, 550025, China.
BMC Evol Biol. 2018 Jul 13;18(1):111. doi: 10.1186/s12862-018-1223-0.
Hirsutella Pat genus, the asexual morphs of the Ophiocordyceps Sung, is globally distributed entomopathogenic fungi, which infect a variety of arthropods, mites and nematodes. The fungal species also have shown potential application in the field of biological control, bio-medicine and food development. Although these fungi are synonymized under Ophiocordyceps, formal taxonomic assignments remain necessary for classification of species in Hirsutella. However, due to the heterogeneity and complexity of Hirsutella genus, more detailed taxonomic and phylogenetic analyses are required to address the following subjects: (1) the relationships between the phialide morphological characteristics and phylogenetic information of Hirsutella with asexual morphs, (2) the origin and evolution of the phialide structure, and (3) host specificity and fungal pathogenicity.
Five typical phialide structures are summarized, in which the variation in phialide characteristics overlaps well with phylogenetic information. A new member of the special twisted neck clade in the Hirsutella-like group, Ophiocordyceps retorta, was reported based on these analyses. The molecular clock calibration analysis based on one fossil record revealed that Hirsutella (asexual morph) species originated from a common ancestor approximately 102 million years ago (Mya) (Early Cretaceous, Lower Albian) and then resolved into two major lineages. One lineage was typically phialidic, which was a larger shape, including H. guyana, H. nodulosa and H. sinensis clades (86.9 Mya, 95% highest posterior density (HPD): 69.1-101.4 Mya). Another main lineage of the phialides was more diversified and smaller than the former, which included H. citriformis and H. thompsonii clades (71.9 Mya, 95% HPD: 41.8-99.6 Mya).
Our results showed that certain phialide characteristics of Hirsutella were phylogenetically informative for two groups of taxa. The differentiation of the phialides structures in the major clades demonstrated a clear evolutionary path of Hirsutella (asexual morph) species, which exhibited two trends depending on the host size. Fungi in one of the groups displayed elongated conidiogenous cells with increased complexity of auxiliary structures from the mycelia. The species in another group reduced the volume of phialides and spores, which might be due to an energy-efficient strategy. These results suggested that a common origin allowed for diversification of given clades into separate niches. The distinct parallel evolutionary path combined with the specific phialides structure might result in the host specificity of Hirsutella (asexual morphs). A direct relationship between Hirsutella (asexual morphs) and the Cretaceous-Tertiary extinction was not found, which suggested that the diversity of phialides is more likely to be caused by long-term environmental adaptation and evolution rather than dramatic extinction events. This evolutionary result might correspond to the background of important biological and geological events in the late Cretaceous occurring near the divergence times of Hirsutella (asexual morphs).
中国被毛孢属的无性型是一种全球分布的昆虫病原真菌，它可以感染多种节肢动物、螨和线虫。这些真菌在生物防治、生物医药和食品开发领域也具有潜在的应用价值。尽管这些真菌被归入中国被毛孢属，但为了对中国被毛孢属的物种进行分类，仍有必要进行正式的分类学鉴定。然而，由于中国被毛孢属的异质性和复杂性，需要更详细的分类和系统发育分析来解决以下问题：（1）中国被毛孢属无性型的分生孢子梗形态特征与系统发育信息之间的关系；（2）分生孢子梗结构的起源和进化；（3）宿主特异性和真菌致病性。
总结了五种典型的分生孢子梗结构，其中分生孢子梗特征的变化与系统发育信息很好地重叠。根据这些分析，报道了一个新的中国被毛孢样群特殊扭曲颈类群成员，即弯颈中国被毛孢。基于一个化石记录的分子钟校准分析表明，中国被毛孢（无性型）物种起源于约 1.02 亿年前（早白垩世，阿尔布期早期）的一个共同祖先，然后分为两个主要谱系。一个谱系的分生孢子梗通常较大，形状呈典型的瓶梗状，包括圭亚那中国被毛孢、中国被毛孢 nodulosa 和中华中国被毛孢等分支（86.9 Mya，95%最高后验密度（HPD）：69.1-101.4 Mya）。另一个主要的分生孢子梗谱系则更为多样化，体积也比前者小，包括中国被毛孢 citriformis 和中国被毛孢 thompsonii 等分支（71.9 Mya，95%HPD：41.8-99.6 Mya）。
我们的结果表明，中国被毛孢某些分生孢子梗特征在两个分类群中具有系统发育信息。主要分支中分生孢子梗结构的分化显示了中国被毛孢（无性型）物种的清晰进化路径，这表现出两种趋势，取决于宿主的大小。其中一个分支的真菌表现出具有增加辅助结构复杂性的伸长的产孢细胞。另一个分支的真菌则减少了分生孢子梗和孢子的体积，这可能是由于能量效率的策略。这些结果表明，一个共同的起源允许特定分支多样化到单独的小生境中。独特的平行进化路径与特定的分生孢子梗结构相结合，可能导致中国被毛孢（无性型）的宿主特异性。中国被毛孢（无性型）与白垩纪-第三纪灭绝之间没有直接关系，这表明分生孢子的多样性更可能是由于长期的环境适应和进化，而不是剧烈的灭绝事件。这一进化结果可能与白垩纪晚期重要的生物和地质事件的背景相对应，这些事件发生在中国被毛孢（无性型）分化时间附近。
