Sex- and Male-Morph-Specific Variation in Brain Mass and Cell Number Scaling in Solitary Centris pallida (Hymenoptera: Apidae) Bees.

Intraspecific variation in behavior is associated with variable brain resource allocation patterns: There is frequently increased tissue investment in discrete regions that support fitness-relevant cognitive abilities. However, the relationships between tissue volume and actual cell numbers have rarely been explored for insects due to methodological hurdles recently addressed via the application of isotropic fractionation. In solitary desert Centris pallida (Hymenoptera: Apidae) bees, there are two major levels of intraspecific variation: sex (males vs. females) and male morph (as a result of alternative reproductive tactics, large morph and small morph males rely on scent or sight, respectively, for mate location). Using isotropic fractionation, we separately analyzed optic lobe (OL) and central brain (CB) cell numbers of males and females to determine the impacts of sex and morph on brain cell allometry. Female bees' brains were bigger and had higher cell numbers and cell densities than males of the same size. In both sexes, total brain cell number increased with brain size, driven by increases in OL cell numbers. Between male morphs, we found that OL masses were relatively larger in small-morph males, consistent with the relationship between body size and OL volumes reported in prior studies. However, small-morph C. pallida males had fewer total cells (as represented by cell nuclei) and reduced cell density, in their OLs. Together, these data suggest that there is intraspecific and brain-region-specific variation in brain cell numbers and that variation in brain tissue volume may not match other levels of neural organization like brain cell numbers/densities.