Testing for intraspecific postzygotic isolation between cryptic lineages ofPseudacris crucifer

Abstract

Phenotypically cryptic lineages appear common in nature, yet little is known about the mechanisms that initiate and/or maintain barriers to gene flow, or how secondary contact between them might influence evolutionary trajectories. The consequences of such contact between diverging lineages depend on hybrid fitness, highlighting the potential for postzygotic isolating barriers to play a role in the origins of biological species. Previous research shows that two cryptic, deeply diverged intraspecific mitochondrial lineages of a North American chorus frog, the spring peeper (Pseudacris crucifer), meet in secondary contact in Southwestern Ontario, Canada. Our study quantified hatching success, tadpole survival, size at metamorphosis, and development time for experimentally generated pure lineage and hybrid tadpoles. Results suggest that lineages differ in tadpole survival and that F1 hybrids may have equal fitness and higher than average mass at metamorphosis compared with pure parental crosses. These findings imply hybrid early life viability may not be the pivotal reproductive isolation barrier helping to maintain lineage boundaries. However, we observed instances of tadpole gigantism, failure to metamorphose, and bent tails in some tadpoles from hybrid families. We also speculate and provide some evidence that apparent advantages or similarities of hybrids compared with pure lineage tadpoles may disappear when tadpoles are raised with competitors of different genetic makeup. This pilot study implies that ecological context and consideration of extrinsic factors may be a key to revealing mechanisms causing negative hybrid fitness during early life stages, a provocative avenue for future investigations on barriers to gene flow among these intraspecific lineages.

Contact zone dynamics during early stages of speciation in a chorus frog (Pseudacris crucifer)

“…In the absence of quantified fitness consequences in hybrid P. crucifer (Stewart and Lougheed, 2013), plausible alternatives for the patterns that we observed require discussion. For example, asymmetrical mtDNA introgression may be caused by local adaptation, mtDNA-nDNA coevolution or selective sweeps of universally favoured mutations (Irwin et al, 2009).…”

“…Hybrid males show gametic (Wang, 2012) and behavioural (alternative mating tactic) dysfunctions (Stewart, 2013), in addition to some observations of higher tadpole mortality in hybrids under stressful developmental periods (Stewart and Lougheed, 2013), although this latter finding requires confirmation.…”

Extensive variation in sperm morphology in a frog with no sperm competition

“…Barriers to fertilization presumably arise late in the speciation process, with some authors positing that selection against gamete wastage should favour the evolution of earlier-acting reproductive barriers [ 51 ]. Premating isolation through both acoustic differences and population-specific female preference have indeed been shown in spring peepers [ 14 ], while postzygotic reproductive barriers, as demonstrated through experimental evidence on spring peeper tadpoles, remains incomplete [ 52 ]. Divergence in spring peeper external morphology is relatively cryptic and no one has yet suggested that these lineages be recognized as incipient species.…”

Can alternative mating tactics facilitate introgression across a hybrid zone by circumventing female choice?

“…Female HE individuals are significantly smaller than their contact zone counterparts (Stewart et al ., ) and female size correlates with fecundity both within (Camargo et al ., ) and among (Prado & Haddad, ) anuran species. Further, preliminary research also suggests tadpole competitive inferiority for HE individuals causing high levels of juvenile mortality (Stewart & Lougheed, ). Taken together, a decrease in HE fecundity and an increase in HE mortality may help to explain a decrease in proportions of Eastern hybrids within the contact zone.…”