This study indicates that constraints of host physiology or genetics have limited opportunities for successful horizontal transfers during reproductive parasite evolution.
The importance of both pre-adaptation and post-emergence evolution may also explain the apparent paradox of constraints on host shifts, even to permissible hosts, despite the vast potential for rapid genetic change in RNA viruses.
Emerging infectious diseases may thus be examples where pathogens escape the evolutionary constraints of particular hosts and enter different evolutionary backgrounds (Cox-Singh 2012; Morse et al. 2012).
We suggest that the observed patterns of life cycle evolution in the genus Brachycaudus, and possibly similar patterns in other genera of the Aphidinae, probably result from a combination of both selection and constraints on host plant choices.
Comparing the results of reconciliation analyses to that of methods investigating simple congruence was important for identifying these constraints on host shifts imposed by host phylogeny.
Any constraints on the behaviour of corporate investors in Africa (such as the Committee on World Food Security's guidelines on land tenure) remain voluntary, while the constraints on host nations become compulsory.
Constraints on host range could arise due to fitness trade-offs (Duffy et al., 2006), differences between hosts in terms of overall quality (Heineman et al., 2008), or intracellular defenses against successfully adsorbed phages (Sieber & Gudelj, 2014).
The conclusion reached here is that the canalized character of HA evolution arises primarily from the two critical parameters in the model (mutation rate, immunological distance created by mutation) rather than from the functional constraints of viral invasion of the host, though the molecule must of course maintain its core activity.
No attempt was made to control for kinship relationships between paired females in this study due to constraints on the availability of hosts combined with the difficulty of differentiating male and female E. nassaui.
2×104 mutagenized phages is 20 times greater than the number of phages used to initiate individual φX174 evolution experiments, allowing us to conclude that genetic constraints prevent the evolution of host generalization in G4 populations.
