A test of adaptive divergence in a newly discovered host association of the soapberry bug Jadera haematoloma on Mexican buckeye, Ungnadia speciosa. Issue 1 (18th November 2021) - Comerford, Mattheau S.
Abstract: Host races represent an important step in the speciation process of phytophagous insects as they reflect the maintenance of genetically divergent host‐associated populations in the face of appreciable gene flow. The red‐shouldered soapberry bug, Jadera haematoloma (Herrich‐Schäffer) (Hemiptera: Rhopalidae), is an oligophagous seed predator with a history of host race evolution on plant associations in the (soapberry) family Sapindaceae. Soapberry bugs are a model group for understanding rapid ecological adaptation to their hosts, and hence good candidates for investigating evolutionary divergence in host associations over short timescales. Here, we describe the recent discovery of Mexican buckeye, Ungnadia speciosa Endl., as a host of J. haematoloma in a region of the Chihuahuan desert including west Texas and southeastern New Mexico, USA. This host differs from J. haematoloma 's previously recorded hosts in the Sapindaceae in seed chemistry, ecology, and phylogeny. The tendency toward rapid, host‐associated adaptations by populations of J. haematoloma and the unique biology of the newly discovered Ungnadia host create the opportunity for potential host race formation, as it overlaps geographically with two previously recorded host plants in this region – the native western soapberry tree, Sapindus saponaria var. drummondii (Hook & Arn.), and the non‐native goldenrain tree, Koelreuteria paniculata Laxm. We explore the possibility of host race formation on Ungnadia ‐associated insects by testing for host‐associated differentiation in morphology and feeding behaviors. We find evidence of differentiation in the length of the mouth parts, which is an ecologically relevant feeding trait between host plant species with larger or smaller seed capsules. This divergence is maintained in the face of potential gene flow by reproductive isolation in the form of habitat isolation, which we detect in host preference trials. Together, our results demonstrate that soapberry bugs associated with this newly discovered host exhibit morphological and behavioral traits consistent with host race formation, but additional work is required to confirm its state along the speciation continuum. Abstract : The red‐shouldered soapberry bug, Jadera haematoloma (Hemiptera: Rhopalidae), is a seed predator with a history of rapid ecological adaptation and host race evolution on plants in the (soapberry) family Sapindaceae. Here we describe a newly discovered regionally limited host association on Mexican buckeye, Ungnadia speciosa, which is chemically, phylogenetically, and ecologically different from the insect's previous known hosts. This expansion of diet breadth has resulted in both morphological and behavioral differentiation in insect phenotype that may lead to a novel host race formation.