The life cycles of many organisms require progression through series of phenotypic transitions that facilitate their activities in different ecological niches. This phenotypic turnover is best exemplified by life cycles that involve complex and abrupt changes, such as the metamorphosis of tadpoles into frogs or the transmission between distinct host species of parasites. Deeply rooted divergence between lineages has often occurred in association with principal life cycle alterations (Wheeler et al., 2001, Reiss, 2002). Therefore, an understanding of life cycle evolution has the potential to inform a more general understanding of patterns of macroevolutionary diversification. Although biologists have made significant progress in understanding life cycle evolution, an inconsistency remains between theory describing life cycle evolution and how life cycle complexity is typically evaluated, particularly in parasites (Auld and Tinsley, 2015, Benesh, 2016).

The fundamental goal of studying life cycle evolution is to understand how and why the phenotypes presented by organisms differentiate as they undergo ontogenetic niche shifts, which are the changes in ecological niche that organisms experience as they develop and age (Werner and Gilliam, 1984). Theory suggests that if the traits presented by different stages share a genetic basis, phenotypic divergence between stages will be relatively constrained because variation that is neutral or beneficial in one niche may be detrimental in another (Ebenman, 1992). However, if the traits expressed by different stages become genetically decoupled, different stages may evolve independently, thus facilitating their differentiation (Ebenman, 1992).

Genetic (and consequently evolutionary) decoupling of at least some traits expressed by different life stages is empirically well supported (Fellous and Lazzaro, 2011, Aguirre et al., 2014, Goedert and Calsbeek, 2019, Schott et al., 2022). Just as transcriptional dissimilarity generates differences between cell types, phenotypic decoupling between life stages is produced by their overall dissimilarity in gene expression (Fellous and Lazzaro, 2011, Herrig et al., 2021, Collet et al., 2023, DuBose and de Roode, 2024). Therefore, a primary driver of differentiation between life stages is specialized pattens of gene expression to particular stages. Conversely, genes that are expressed broadly at consistent levels across an organism’s life cycle contribute little to differentiation between life stages. Increased temporal specificity in patterns of gene expression have been linked to gene duplication and increased rates of molecular evolution, which shows general consistency with theory on life cycle evolution and our understanding of how cell and tissue differentiation evolves (Williams et al., 2023, DuBose and de Roode, 2024).

Although biologists are beginning to understand the genetic and transcriptional basis of trait decoupling across ontogeny, how life cycle complexity is typically defined in ecological and evolutionary studies typically relies on binary classification systems. From a zoological perspective, which much theory on life cycle evolution is based on, life cycles are considered complex if they progress through two or more discrete stages and are otherwise considered simple (Istock, 1967, Moran, 1994). Similarly, parasites are considered to have complex life cycles if they progress through two or more host species and simple if they occupy a single host species. In this, parasites that utilize more intermediate host species are generally considered to have more complex life cycles (Combes, 2001, Auld and Tinsley, 2015). These perspectives are conceptually similar in that they consider abrupt ontogenetic niche shifting indicative of greater life cycle complexity (Benesh, 2016). However, many parasites can experience significant niche shifts in a single host, particularly those that move between different tissues (Haldane, 1932) or, as we explore here, utilize hosts that undergo a metamorphosis.

More generally, theory suggests that the greater the dissimilarity between niches occupied by different stages, the more their phenotypes will diverge, given genetic independence (Ebenman, 1992). Here we apply this concept towards describing the transcriptional dynamics of Ophryocystis elektroscirrha, a protozoan parasite that infects the monarch butterfly Danaus plexippus as its single host (McLaughlin and Myers, 1970). We found evidence that O. elektroscirrha undergoes significant functional differentiation throughout the monarch metamorphosis. We then found support for the prediction that greater dissimilarity between niches occupied by different life stages corresponds with greater dissimilarity between stages, as patterns of transcriptional dissimilarity across the O. elektroscirrha life cycle aligned with transcriptional turnover between monarch life stages. Finally, we found that transcriptional decoupling between O. elektroscirrha life stages exceeded that of its host, but that D. plexippus exhibited greater total transcriptional turnover. Given that the monarch life cycle is considered complex while that of the parasite is considered simple, these findings suggest a need to reevaluate how parasitology and evolutionary biology describe and theorize about life cycle complexity.