Taxonomic And Phylogenetic Studies Of Two Types Of Small Green Algae In China

Hydrodictyaceae and subfamily Makinoellodeae are two typical small green algae in the phylum Chlorella,which are often distributed in freshwater bodies such as rivers and lakes,and sometimes even found in subatmospheric habitats.In this study,samples were widely collected from various provinces in China,and algae strains were purified and cultured by capillary separation and coating plate method.Morphology,ultrastructural observation and phylogenetic methods were used to systematically classify and identify algae strains.Subsequently,the ITS2 secondary structures and taxa below the phylogen analysis section were introduced to solve the taxonomic problems in the taxonomic system and lay a foundation for the study of the diversity of small green algae in China.The results of the study are as follows:(1)Isolated and pured 82 strains from Hydrodictyaceae,10 species and 10 variants of 8 genera were identified,including Pediastrum,Monactinus,Lacunastrum,Pseudopediastrum,Parapediastrum,Stauridium,Hydrodictyon,Sorastrum.The 226 genes were sequenced.By combining morphological and phylogenetic analysis,it was found that Hydrodictyaceae with various forms were unreliable by morphological identification alone.The phylogenetic tree results can also show that there are often low support values within Hydrodictyaceae,the location of many strains is ambiguous,and the polyphyletic phenomenon is serious.This study initially solves the taxonomic problems,and adds new members,supporting the existence of an evolutionary route from twodimensional to three-dimensional from the genus Pediastrum to the genus Hydrodictyon and the genus Sorastrum.The ITS2 secondary structure showed the difference of CBCs(including semi-CBCs)between different species,which could be used as a classification criterion between genera and species,supporting the above classification results and providing a reliable basis for the accurate classification of the family Hydrodictyaceae.(2)Isolated and pured 17 strains from the subfamily Makinoellodeae,6 genera and9 species were identified,including Makinoella,Willea,Rhoicocystis,Elongatocystis,Ecballocystopsis,Neglectella.2 new genera were described,including Elongatocystis and Rhoicocystis.3 new species were erected,including Makinoella parva,Elongatocystis gelatinosum and Rhoicocystis nephrocytioides.The 39 new genes were sequenced.By combining with morphological and phylogenetic analysis,it was found that the cell volume of the subfamily Makinoellodeae was relatively large,and the mother cell wall was easy to expand,which can wrap the daughter cells to form a colony.The structural system is rich,including single cell,small population,dendritic branching coenobium and pseudofilaments.The results of phylogenetic analysis also showed that the taxonomic position of algal plants was basically consistent in both single-gene and polygenic phylogenetic trees.Makinoellodeae cluster into a large branch on the phylogenetic tree.From the base to the top,there is an evolutionary route that the cell volume gradually decreases,the number of chloroplasts gradually decreases,and the structural system types tend to be simple.The chloroplast genomic analysis also supports the evolutionary route within Makinoellodeae,which is helpful for further analysis of the evolution of Oocystaceae from aquatic to subatmospheric habitats.In summary,through the systematic classification of two small green algae,Hydrodictyaceae and Makinoellodeae,the relevant taxonomic problems were preliminarily solved,the richness of species was greatly improved,and it also provided certain reference value for the distribution and diversity protection of green algae in China.