Facultative parasites as evolutionary stepping-stones towards parasitic lifestyles

Abstract

Parasites and parasitic lifestyles have evolved from free-living organisms multiple times. How such a key evolutionary transition occurred remains puzzling. Facultative parasites represent potential transitional states between free-living and fully parasitic lifestyles because they can be either free-living or parasitic depending on environmental conditions. We suggest that facultative parasites with phenotypically plastic life-history strategies may serve as evolutionary stepping-stones towards obligate parasitism. Pre-adaptations provide a starting point for the transition towards opportunistic or facultative parasitism, but what evolutionary mechanism underlies the transition from facultative to obligate parasitism? In this Opinion Piece, we outline how facultative parasites could evolve towards obligate parasites via genetic assimilation, either alone or in combination with the Baldwin effect. We further describe the key predictions stemming from each of these evolutionary pathways. The importance of genetic assimilation in evolution has been hotly debated. Studies on facultative parasites may not only provide key insights regarding the evolution of parasitism, but also provide ideal systems in which to test evolutionary theory on genetic accommodation.

First report of a gastropod parasitic nematode Phasmarhabditis californica (Nematoda: Rhabditidae) in Alberta, Canada

“…Research on the infectivity and host specificity of this Canadian strain against pestiferous and native gastropods is needed to determine its potential as a biological control agent in Canada. Furthermore, Phasmarhabditis sp., a facultative parasite, could also be employed as new genetic model nematodes to study the evolution of parasitism (Andrus and Rae, 2019;Luong and Mathot, 2019).…”

“…The status of slug and snail as pests has been growing globally, yet the widespread use of chemical control measures is less than ideal (Bailey, 2002;Wilson and Rae, 2015). Baited pellets and liquid sprays containing methiocarb, metaldehyde, iron phosphate, and sodium ferric EDTA sometimes fail to provide adequate protection for crops (Hata et al, 1997;Bailey, 2002). These products are also poisonous to birds, mammals, and invertebrates (Purves and Bannon, 1992;Fletcher et al, 1994;Bailey, 2002).…”

Phoresy in animals: review and synthesis of a common but understudied mode of dispersal

“…(2001) to hypothesize an evolutionary transition from phoresy to parasitism to mutualism in this system. In phoretic nematodes, necromeny may have been key in the transition to parasitism for some species (Kiontke & Sudhaus, 2006; Hong & Sommer, 2006 b ; Dieterich & Sommer, 2009; Luong & Mathot, 2019). Necromeny is the process in which dauer larvae of nematodes attach to a live host and, after the host dies (from non‐phoretic causes), feed on microbes on the decaying carcass.…”

Phylogenetic evidence for the invasion of a commercialized European Phasmarhabditis hermaphrodita lineage into North America and New Zealand

“…The predatory snail Euglandina rosea has been introduced to Pacific and Indian Ocean islands for the purpose of controling giant African land snails (Lissachatina fulica), but has turned out to be ineffective against the target invasive species and damaging to island-endemic gastropods instead [8]. These classic 'biocontrol gone awry' and similar stories have motivated the development of rigorous regulatory policies surrounding efforts to develop new biocontrol initiatives, requiring careful environmental impact analyses and other forms of scientific scrutiny prior to biocontrol agent deployment [9][10][11][12].…”