The endocrine-genetic basis of life-history variation: The relationship between the ecdysteroid titer and morph-specific reproduction in the wing-polymorphic cricket Gryllus firmus

The hormonal basis of variation in life-history traits is a poorly studied topic in life-history evolution. An important step in identifying the endoc rine-genetic causes of life-history variation is documenting statistical an d functional associations between hormone titers and genotypes/phenotypes t hat vary in life-history traits. To this end, we compared the blood ecdyste roid titer and the mass of the ovaries during the first week of adulthood a mong a flight-capable morph and two flightless morphs of the wing-polymorph ic cricket Gryllus firmus. Ecdysteroids are a group of structurally related hormones that regulate many important aspects of reproduction in insects. Both the ecdysteroid titer and ovarian mass were significantly higher in ea ch of two flightless morphs compared with the flight-capable morph througho ut the first week of adulthood. Genetically based differences in the ecdyst eroid titer and ovarian mass between morphs from different selected lines w ere similar to phenotypically based differences among morphs from the same control (unselected) lines. By day 7 of adulthood, ovaries were typically 2 00-400% larger and the ecdysteroid titer was 60-300% higher in flightless v ersus the flight-capable morph. In addition, highly significant, positive, phenotypic correlations were observed between the ecdysteroid titer and ova rian mass in pooled samples of the two flightless and Eight-capable cricket s from control lines or from selected lines. The ecdysteroid titer was suff iciently elevated in the flightless morphs to account for their elevated ov arian growth. This is the first direct documentation that naturally occurri ng phenotypes/genotypes that differ in early fecundity, a key life-history trait, also differ phenotypically and generically in the titer of a key rep roductive hormone that potentially regulates that trait.