Bridging reproductive and microbial ecology: a case study in arbuscular mycorrhizal fungi

Carlos A Aguilar-Trigueros,Stefan Hempel,William K Cornwell,Jeff R Powell,Matthias C Rillig

doi:10.1038/s41396-018-0314-7

Abstract

Offspring size is a key trait for understanding the reproductive ecology of species, yet studies addressing the ecological meaning of offspring size have so far been limited to macro-organisms. We consider this a missed opportunity in microbial ecology and provide what we believe is the first formal study of offspring-size variation in microbes using reproductive models developed for macro-organisms. We mapped the entire distribution of fungal spore size in the arbuscular mycorrhizal (AM) fungi (subphylum Glomeromycotina) and tested allometric expectations of this trait to offspring (spore) output and body size. Our results reveal a potential paradox in the reproductive ecology of AM fungi: while large spore-size variation is maintained through evolutionary time (independent of body size), increases in spore size trade off with spore output. That is, parental mycelia of large-spored species produce fewer spores and thus may have a fitness disadvantage compared to small-spored species. The persistence of the large-spore strategy, despite this apparent fitness disadvantage, suggests the existence of advantages to large-spored species that could manifest later in fungal life history. Thus, we consider that solving this paradox opens the door to fruitful future research establishing the relationship between offspring size and other AM life history traits.

Highlights

Offspring size is a highly variable trait in living organisms
In our subsequent analysis, we used spore-size diameter estimates calculated from inner-range means because we believe that these inner-range values are more representative of the spore size of a given species
The trade-off between spore output and spore size we show here (Fig. 3) suggests that arbuscular mycorrhizal (AM) fungal spores are costly and increases in individual spore size translate into higher energy and resource demands, which results in reduction of fitness for the parental mycelia

Summary

Introduction

Offspring size is a highly variable trait in living organisms. For example, in birds, egg size ranges from 1.2 cm in length for hummingbirds to 14.4 cm for the common ostrich (major diameter length) [1]. This large variation in offspring size is not random or inconsequential: it has important ecological and evolutionary meaning because offspring size—a proxy for the quantity of resources that parents allocate to each individual offspring—greatly influences the survival and fitness of the offspring [5, 6]. From the perspective of inclusive fitness theory [12], offspring size is a interesting trait because its variation is regulated by selection pressure acting on both the parental and progeny generations [9]

Methods

Results

Discussion

Conclusion