Evaluating sexual dimorphism in the ambrosia beetle Xyleborus affinis (Coleoptera: Curculionidae) using geometric morphometrics
Abstract
Species within genus Xyleborus Eichhoff (Curculionidae: Scolytinae) live in a feeding symbiosis with ambrosia fungi and present inbreeding polygyny with highly biased sex ratios and sexual dimorphism in size. Here we evaluate the expression of sexual dimorphism, the allometric effect on shape variation, and the morphological integration of 2 body structures (elytra and pronotum) in Xyleborus affinis (Eichhoff) (Curculionidae: Scolytinae) under controlled growth conditions (26 °C, 60% RH, and continuous darkness), using geometric morphometrics. Differences between sexes from statistical multivariate tests indicate the presence of sexual size and shape dimorphism in both structures, with the largest differences and shape variability presented in the pronotum. We found significant developmental integration of both structures (morphological covariation) and no significant differences in allometric trends between sexes. The results suggest a specific allometric pattern in X. affinis that could be constrained by genetic or phylogenetic factors. However, quantification of shape and size variation could help to determine the differential effects of environmental stress between sexes during growth in this species. This approach could be valuable for monitoring populations of other Xyleborus species living in contrasting environmental conditions, given their wide geographical distributions and the vast amount of host plant species with which they interact.
Resumen
Las especies del género Xyleborus Eichhoff (Curculionidae: Scolytinae) viven en simbiosis de alimentación con hongos ambrosiales, y que presentan endogamia poligínica, con proporciones sexuales sesgadas y dimorfismo sexual en tamaño. Usando un protocolo de morfometría geométrica evaluamos la expresión del dimorfismo sexual, el efecto alométrico sobre la variación de la forma y la integración morfológica de dos estructuras corporales (élitro y pronoto) en X. affinis (Eichhoff) (Curculionidae: Scolytinae) en condiciones de crecimiento controlado (26 °C, 60% HR y oscuridad continua). Los análisis soportan diferencias estadísticas entre los sexos e indican la presencia de dimorfismo sexual en tamaño y forma de ambas estructuras, con mayores diferencias y alta variabilidad de la forma del pronoto. No hubo diferencias significativas en las tendencias alométricas entre los sexos y se encontró una integración significativa (covariación morfológica) en el desarrollo de ambas estructuras. Los resultados sugieren un patrón alométrico específico para X. affinis que podría estar limitado por factores genéticos o filogenéticos. La cuantificación de la variación de la forma y el tamaño podría ayudar a explorar los efectos diferenciales del estrés ambiental entre los sexos durante el crecimiento en esta especie. Este enfoque podría ser valioso para monitorear poblaciones de otras especies de Xyleborus que viven en condiciones ambientales contrastantes, dada su amplia distribución geográfica, y que crecen en diferentes especies de plantas hospederas.
Key Words: ambrosia fungi; morphological variation; secondary sexual characters; Scolytinae; wood boring beetles
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