Abstract
Root-knot nematodes are obligate parasites of a wide range of plant species and can feed only on the cytoplasm of living plant cells. In the absence of a suitable plant host, infective juveniles of strain VW9 of the Northern root-knot nematode, Meloidogyne hapla, when dispersed in Pluronic F-127 gel, aggregate into tight, spherical clumps containing thousands of worms. Aggregation or clumping behavior has been observed in diverse genera in the phylum Nematoda spanning free-living species such as Caenorhabditis elegans as well as both plant and animal parasites. Clumping behavior differs between strains of M. hapla and occurs with other species within this genus where strain-specific differences in clumping ability are also apparent. Exposure of M. hapla juveniles to a gradient formed using low levels of cyanide promotes formation of clumps at a preferred cyanide level. Analysis of F2 lines from a cross of M. hapla strains that differ in clump-forming behavior reveals that the behavior segregates as a single, major locus that can be positioned on the genetic map of this nematode. Clumping behavior may be a survival strategy whose importance and function depend on the niche of the nematode strain or species.
Highlights
Root-knot nematodes (Meloidogyne spp.) are obligate endoparasites of a wide range of plant species and globally cause large crop yield losses [1,2]
Nematodes suspended in Pluronic F-127 (PF-127) gel, rather than being limited to the surface as with agar plates, can respond to signals perceived in three dimensions
We previously found that infective J2 of three different root-knot nematode species (M. hapla strain VW9, M. javanica strain VW4, and M. incognita strain 557R) aggregated into tightly packed spherical clumps by two days after being uniformly suspended in PF-127 [6]
Summary
Root-knot nematodes (Meloidogyne spp.) are obligate endoparasites of a wide range of plant species and globally cause large crop yield losses [1,2]. The infective stage, which is the second stage juvenile (J2), hatches from an egg in the soil and must find a host and establish a feeding site in order to survive. The J2 typically enters the host root in the zone of elongation migrates to the vascular cylinder to establish a feeding site and complete its life cycle. Many nematodes, including most plant parasitic species, inhabit the soil for at least part of their life cycle. Movement of nematodes to root tips can be monitored as well as changes in behavior as they approach the zone of elongation where they penetrate the root. Because stable chemical gradients form in PF-127, we were able to show that root-knot nematode J2 accumulate at pH between 4.5 and 5.5, consistent with the low pH of the zone of elongation relative to other regions of the root [7]
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