Analysis of nematode-endosymbiont coevolution in the Xiphinema americanum species complex using molecular markers of variable evolutionary rates

Authors: HOWE, D - Oregon State University; SMITH, M - Oregon State University; TOM, D - Oregon State University; BROWN, A - Texas Tech University; Peetz, Amy; Zasada, Inga; DENVER, D - Oregon State University

Submitted to: Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2018
Publication Date: 12/7/2018
Citation: Howe, D.K., Smith, M., Tom, D.M., Brown, A.M., Peetz, A.B., Zasada, I.A., Denver, D.R. 2018. Analysis of nematode-endosymbiont coevolution in the Xiphinema americanum species complex using molecular markers of variable evolutionary rates. Nematology. 21(5):533-546. https://doi.org/10.1163/15685411-00003233.
DOI: https://doi.org/10.1163/15685411-00003233

Interpretive Summary: Plant-parasitic nematodes are microscopic roundworms that have been shown to cause significant yield loss to a range of crops. The focus of this research was the dagger nematode which can cause direct damage to plants and also transmit plant viruses to crops such as grapes, raspberries, and blueberries. These nematode harbor a bacteria internally, Xiphinematobacter, which appears to be important to the nematode for essential nutritional requirements. This research was conducted to explore the development of this relationship (coevolution) between the nematode and bacteria. DNA was extracted from the dagger nematode populations, and several regions of the nematode and bacterial DNA were analyzed. It was discovered that the nematode and the bacteria have evolved together and that this nematode/bacterial relationship could be defined by six distinct groups. These results are significant because it demonstrates the power of analyzing several DNA regions to understand complex relationships among nematodes and bacteria. This research will be used by scientists to continue to explore the relationship between dagger nematodes and Xiphinematobacter and to utilize this information to determine if there is an opportunity to control the nematode by targeting the bacteria inside the nematode.

Technical Abstract: Bacterial symbioses play important roles in shaping diverse biological processes in nematodes, and serve as targets in nematode biocontrol strategies. Nematodes in the genus Xiphinema undergo symbiosis with the bacterial endosymbiont Xiphinematobacter spp. These endosymbionts are vertically transmitted and hypothesized to function as metabolic mutualists. We expanded upon recent research investigating patterns of coevolution – then between Xiphinema spp. and Xiphinematobacter spp. by focusing on nematodes in the Xiphinema americanum species complex, utilizing two symbiont genetic markers of varying evolutionary rates. Phylogenetic analysis of nematode mitochondrial DNA (mtDNA) revealed five strongly supported major clades. Analysis of slow-evolving 16S ribosomal DNA (rDNA) in bacterial symbionts resulted in a phylogenetic topology composed of three major clades that grouped taxa in a fashion highly congruent with the nematode mtDNA topology. A faster evolving protein-coding gene encoded in the symbiont genome (nad) provided more phylogenetic resolution with six well-supported clades, also congruent with the nematode mtDNA tree topology. Coevolutionary analysis using the program Jane further supported strong overall coevolution between members of the X. americanum species complex and their Xiphinematobacter spp. endosymbionts. Our results reinforced recent studies suggesting extensive coevolution between Xiphinema spp. and their vertically transmitted endosymbionts Xiphinematobacter spp. and illustrated the advantages of including genetic markers of varying evolutionary rates in coevolutionary and other phylogenetic studies.