Many plant-sap-feeding insects have maintained a single, obligate, nutritional symbiont over the long history of their lineage. This senior symbiont may be joined by one or more junior symbionts that compensate for gaps in function incurred through genome-degradative forces. Adelgids are sap-sucking insects that feed solely on conifer trees and follow complex life cycles in which the diet fluctuates in nutrient levels. Adelgids are unusual in that both senior and junior symbionts appear to have been replaced repeatedly over their evolutionary history. Genomes can provide clues to understanding symbiont replacements, but only the dual symbionts of hemlock adelgids have been examined thus far. Here, we sequence and compare genomes of four additional dual-symbiont pairs in adelgids. We show that these symbionts are nutritional partners originating from diverse bacterial lineages and exhibiting wide variation in general genome characteristics. Although dual symbionts cooperate to produce nutrients, the balance of contributions varies widely across pairs, and total genome contents reflect a range of ages and degrees of degradation. Most symbionts appear to be in transitional states of genome reduction. Our findings support a hypothesis of periodic symbiont turnover driven by fluctuating selection for nutritional provisioning related to gains and losses of complex life cycles in their hosts.

Background: Many topics in evolutionary biology have been the focus of little research about student thinking and learning. This lack of research limits the evidence base on which instructors can draw to inform their teaching. A key starting place for education research about evolutionary topics is determining what concepts are important for undergraduates to learn. This work develops a community-informed list of key concepts about speciation. Speciation is commonly taught in undergraduate biology education, yet has been the focus of almost no research on teaching and learning. We gathered input from over 110 evolution educators and speciation researchers to create a comprehensive list of speciation concepts for undergraduate education. Results: The community-informed list includes 24 concept statements organized within 4 overarching concepts. At least 80% of experts rated these statements as scientifically accurate and clear. Over 90% of experts rated the statements as important or somewhat important for a graduating senior in biology to understand. Conclusions: This list provides a foundation for both education researchers and evolution educators. Education researchers who investigate student thinking and who develop research-based measurement tools can use this list to determine key concepts on which to focus their future work. Educators can use this list to guide the development of learning objectives for speciation instruction. Future work should investigate what concepts are reasonable for an undergraduate to master in a 4-year degree.