Convergent patterns of body size variation in distinct parasite taxa with convergent life cycles

Abstract

Aim: Interspecific variation among metazoans often follows a latitudinal pattern, with species at higher latitudes being larger bodied than related species from lower latitudes (Bergmann's rule). For parasitic species, body sizes within any higher taxon are often correlated with the body sizes of their hosts (Harrison's rule). Whether temperaturedriven latitudinal effects or host-driven resource constraints act independently or additively to shape interspecific variation in parasite body sizes remains unknown. We take a comparative approach to test the effects of latitude and host body size on parasite body sizes in two taxa of parasitic worms showing convergent life cycles. Location: Global.Time period: Contemporary.Major taxa studied: Hairworms (Nematomorpha) and mermithids (Nematoda) parasitic in arthropods. Methods:With 223 records for mermithids and 258 for nematomorphs world-wide, we used linear mixed effects models to test the effects of latitude and host body size on parasite length, intraspecific length variation, parasite egg diameter and variation in egg diameter. Furthermore, we modelled parasite length with local mean annual temperature as the predictor instead of latitude, as a direct test of underlying mechanisms. All models took into account host and parasite taxonomic structure within the datasets.Results: For both taxa, host body size was clearly the main determinant of parasite body length, with neither latitude nor local temperature (annual mean or range) having an effect. No predictor affected intraspecific length variation, whereas egg diameter was positively associated with parasite length, and variation in egg diameter was negatively associated with latitude.Main conclusions: Our results support a strong role for host traits in shaping the evolution of parasite body sizes (Harrison's rule), but no role for latitude (Bergmann's rule), although these parasites infect ectothermic hosts. At a mechanistic level, the evolutionary driving force of external temperature on parasite physiology seems to be eclipsed by the availability of resources from the host.

Latitudinal gradients in body size and sexual size dimorphism in fleas: males drive Bergmann's pattern

“…Given that a host is the ultimate resource for a parasite, not only as a food source but also as a site for living and reproducing, spatial variation in parasite body size has often been considered in conjunction with either host body size (e.g. Ni et al 2021) or its geographic distribution (e.g. Randhawa & Poulin 2009).…”

“…To obtain a single value characterizing the body size of a species, we used the median of the average male and average female body size. To characterize intraspecific body size variation, we followed Ni et al (2021) and used absolute difference in the body length between the largest and the smallest flea specimen for each species either combined and subsequently taking median value for males and females or separately for males and females.…”

“…Dallas et al 2019), or no relationship whatsoever (e.g. Poulin 1997a;Ni et al 2021) (see also Poulin 2021 for review). For example, in only 7 of 15 comparative analyses, searching for the body size-latitude pattern in parasites and reviewed by Poulin (2021), was the relationship found to be positive.…”

Modeling the geographical distributions of Chordodes formosanus and its mantis hosts in Taiwan, with considerations for their niche overlaps

“…These results may suggest that the Japanese boxing mantis is more important as a host than previously thought. However, it must be noted that, in horsehair worms, host size influences the adult length (Ni et al, 2021 ); the latter is known to positively influence the fertility in these animals (Hanelt, 2009 ). C. formosanus individuals from Japanese boxing mantises tend to be smaller in size than that of those from Taiwanese giant mantises (Chiu et al, 2017 ; M. De Vivo, pers.…”

“…These results may suggest that the Japanese boxing mantis is more important as a host than previously thought. However, it must be noted that, in horsehair worms, host size influences the adult length (Ni et al, 2021); the latter is known to positively influence the fertility in these animals F I G U R E 2 Binary range map for Chordodes formosanus, with the presence points used for modeling. The green area represents where the animal should be present.…”