Genetic Basis For The Formation Of Long Egg Filaments In Colocasioma Gigantea Group Based On Comparative Genomics

The gigantea species group of the genus Colocasiomyia was established recently by Fartyal et al consists of seven described species,i.e.,C.gigantea,C.sindapsae,C.rhaphidophorae,C.longivalva,C.hailini,C.longifilamentata,C.yini.Species of this group obligateorye pollinator of their host plants of the subfamily Monsteroideae,family Araceae.Moreover,In the genus Colocasimyia,the gigantea group is the only lineage adapted to tuse the inflorescences of Monsteroideae.Our field observation revealed that all members of C.gigantea group lay their eggs in the slot between the pistils of the host inflorescence and are characterized by long filaments.The filaments serves as respiratory organ on the anterior back of the egg shell,as an important structure of the egg shell.There are studies that comparison of the eggs between species of the Drosophila melanogaster and C.gigantea group species,revealed significant longer(2-5 times)filament length in the former than in the latter.The peculiar reproductive behavior and significantly longer filaments in the C.gigantea group may be associated with adaptation to specific ovipositing sites(into the pistil slits)and specific floral traits.No relevant report has been seen that the formation of long filaments of the Colocasiomyia and adaptation to specific floral traits.The formation of filaments of the C.gigantea group and the adaptive genetic mechanism provides a important information and new understanding for the adaptive evolution.In this study,comparative genomics were used to study the C.gigantea group.First,using the Real-time single molecule sequencing technology,we get two highquality genomes(the genome sizes 364.2Mb,297.2Mb,respectively;the Contig N50 are 6.6Mb and 10.6Mb,respectivley).The results showed that the genome size of C.gigantea group was 260Mb-360Mb,larger than that of other Drosophila species(130Mb-210Mb).On genome structure,the C.gigantea group have higher proportion of DNA elenments and LINE than the other Drosophila species,and its abundant repeat sequences constitute big genomes with unique adaptation.Then,comparative genomics were used to 4 species include C.rhaphidophorae,C.longivalva and unpublished genomic data for C.hailini,C.longifilamentata to analyze the gene family and positive selection genes.The analysis results showed that based on the 9 species genome data,a total of 127,320 longest transcripts and protein sequences and 7,977 gene families were obtained after clustering.365 gene families were detected in the ancestral branch of C.gigantea group.Among them,19 of the P<0.05 extended gene families contained 695 genes.After the GO enrichment and KEGG enrichment of these genes,it was found that the enrichment of these genes was mainly related to functions such as microtubulin binding and cytoskeleton protein.Meanwhile,phylogenetic tree of ML was constructed based on 1670 one-to-one homologous genes of the 9 species,and the support rate of all nodes was 100.Based on phylogenetic tree,the positive selection analysis of the ancestral branch of the C.gigantea group and it was found that 118 genes were selected.Through enrichment analysis,it was found that these genes were mainly related to cell stress response,embryonic epithelial cell development,mRNA metabolism and other functions.In combination with candidate genes and pathways,we speculate that the phenotype may be caused by the expression increased of the Ran GTP protease and EB1 which may promote epithelial cells of the formation of the spindle,accelerate the process of the cell cycle,cell proliferation.It makes the structure of the filament extension,thus forming specialized long filaments.At the same time,the mutation of Shark and Slipper in the positive selection resulted the structure of the protein change which in regulated the JNK signaling pathway and promot the embryonic development process and formation of specialized egg filaments.The discovery of these genes lay the foundation for the functional experiment to reveal the mechanism of C.gigantea group obligate flower visiting through specialized long filaments.In summary,the result reveals the adaptive genetic mechanism of C.gigantea group and the formation of specialized long filaments at the genome level.Its providing a new understanding for the adaptive evolution of specific pollinators.