Generational shape shifting: changes in egg shape and size between sexual and asexual generations of a cyclically parthenogenic gall former

Abstract

Successive generations of multivoltine species experience selection specific to the spatiotemporal environments encountered that may lead to adaptive divergence in reproductive traits among generations. To compare reproductive effort within and between generations, appropriate volumetric models, selected on the basis of the analysis of egg shape, are required to estimate the sizes (volumes) of individual eggs. We assessed the shape and estimated the volume of individual eggs produced by the temporally and spatially segregated sexual and asexual generations of the gall former, Belonocnema treatae Mayr (Hymenoptera: Cynipini: Cynipidae). Egg shape, indexed as the difference between the polar and equatorial axes of the ellipsoidal eggs, was independent of egg size, but differed between generations. The relationship of egg shape and female body size within and between generations confirmed that egg shape is an intrinsic property of each generation. Generational differences in egg shape then informed the selection of volumetric models to estimate egg size. We modeled asexual generation eggs as both spheres and prolate spheroids, and sexual generation eggs as both cylinders and prolate spheroids. Choice of volumetric model changed estimates of egg size within the asexual generation by 23% and within the sexual generation by 50%. Comparisons between generations based on the above models produced estimated differences in egg volume that ranged from 16 to 114%. In both generations, a prolate spheroid was the most parsimonious model of egg volume. Based on this model, sexual generation eggs averaged 43% larger than asexual generation eggs. The increased size of sexual eggs was achieved via conservation of the egg's equatorial axis and elongation of the polar axis. The shift in egg shape between sexual and asexual B. treatae is the first documented dimorphism in an egg characteristic expressed between generations of a cyclically parthenogenic organism.

Asymmetric, but opposing reductions in immigrant viability and fecundity promote reproductive isolation among host‐associated populations of an insect herbivore

“…Choosing independent measures of viability and fecundity are critical for comparison within a system or between other systems (Porter and Benkman 2017). The length of each tibia was digitally measured by using a LEICA M125 microscope at 100X magnification and LAS v4.4 software (Hood and Ott 2011). In total, we measured tibia length and estimated fecundity for 991 individuals distributed across sites, host plants, and treatments (Table S3).…”

“…In addition, we counted the number of galls, defined as threedimensional tumors-like structures, on each branch/experimental tree and used this information to estimate the proportion of oviposition events that resulted in gall establishment (a direct measurement of wasp survival at later life stages). We did not include the number of wasps produced per gall as a measurement of survival because the emergence rate of wasps in the laboratory is lower (< 20%) than natural populations when protected from natural enemies (∼60%) (Egan and Ott 2007;Hood and Ott 2009).…”

A test of trade-offs in dispersal and reproduction within and between a sister species pair of specialist insect herbivores

“…For each individual wasp, we measured right hind tibia length, right forewing length, and the length, width, and height of the abdomen. In Hymenoptera, the length of the right hind tibia has previously been shown to be correlated with overall body size (Rogers et al 1976;Honěk 1993;Hood et al 2011Hood et al , 2017. To measure tibias and forewings accurately, they were removed from the body and slide mounted prior to measuring.…”

“…Species of Belonocnema are pro-ovigenic, where the ovaries contained within the abdomen house the full complement of eggs at the time of adult emergence (Hood and Ott 2010). Hence, a count of the number of eggs per female provides an estimate of potential maximum lifetime fecundity while measurements of individual egg size provide an estimate of reproductive investment per egg (Hood and Ott 2011). The product of total egg number × mean egg size provides an estimate of potential 'total reproductive effort', which we estimated for each female.…”

“…To count the number of eggs and measure egg size, the abdomen was removed and placed in a depression slide containing a 1:1:13 acetic acid, glycerol, and water solution for at least 20 minutes to loosen ovarian follicular tissue binding eggs to the ovaries and abdomen wall (Hood and Ott 2011). At this time, the exoskeleton was removed, then the abdomen was dyed with a single drop of 1% methylene blue to better visualize the eggs.…”