The ecological function of insect egg micropyles

Abstract

Insect egg micropyles are openings through the chorion allowing sperm entry for fertilisation. Micropyles are diverse structures showing remarkable variation in number, spatial arrangement and physical structure across extant insect orders. Despite being almost ubiquitous across insects, they have received little attention. As key morphological features of an immobile life stage, it is plausible that part of the diversity exhibited by micropyles is adaptive, supporting other egg structures during embryo development. So, while egg fertilization is the primary function of micropyles, they could aid embryo development and be shaped by natural, as well as sexual selection. Here I first used ancestral reconstruction to investigate micropyle presence, number and variation in primitive insects. Then, I used phylogenetic comparative analyses to explore the ecological function of micropyle number. I hypothesised that micropyle number correlated with (1) aeropyle presence facilitating oxygen exchange; (2) aquatic oviposition supporting development in water and is influenced by (3) critical bioclimatic variables. Across 24 hexapod orders, the most likely ancestral state was one or two micropyles, interspecific variation was high and intraspecific variation was low. Mean micropyle number ranged from zero in Entognatha, Strepsiptera and Thysanoptera to 120 in Panstrongylus geniculatus, Hemiptera, and over 100 on average for Apidae, Hymenoptera. Micropyle number was strongly positively related to (1) egg size, with larger eggs having more micropyles; (2) the presence of aeropyles; (3) annual precipitation, with eggs developing in habitats with low annual precipitation exhibiting fewer micropyles; and (4) negatively related to micropyle width, insect eggs having fewer larger micropyles or numerous smaller ones. However, aquatic oviposition did not affect micropyle number. Overall these findings point to an adaptive ecological function of egg micropyles in addition to their primary fertilisation function. This is consistent with the hypothesis that micropyles aid embryo survival, and so this almost‐ubiquitous trait across insects is shaped by sexual and natural selection pressures during this critical life stage. Read the free Plain Language Summary for this article on the Journal blog.

Differences in the permeability and function of eggshell in different-aged eggs of Frankliniella occidentalis (Thysanoptera: Thripidae)

“…Differences in the permeability of insect eggs is mostly determined by the structure of the eggshell and epidermis. As the rst protective layer for communication with the environment, the eggshell during the embryonic period has important physiological functions, such as protecting the embryo, maintaining gas exchange, preventing water loss, preventing pathogens from entering, and assisting sperm to enter the egg [29][30][31] . The current results showed that eggshells can reduce penetration by insecticides.…”

Coevolution between eggs and sperm of insects

“…We first combined published datasets of sperm length (SpermTree: [38]), insect egg sizes [40] and the number of insect micropyles [39]. As yet, no study has brought together data on micropyle width.…”

“…As yet, no study has brought together data on micropyle width. We added to the data of Iossa [39], using published scanning electron microscope (SEM) micrographs to measure the external diameter of the micropyle external pore of insect species [41] contained within SpermTree (figure 1 a,b ). Some species of insect have heteromorphic sperm (two or more sperm types), e.g.…”

“…Given that selective evolutionary pressures that directly shape egg traits, may indirectly influence sperm traits, we are probably not accounting for all variation in sperm diversity. To address this gap, we collated data on sperm length [38], egg size [9] and micropyles [39] alongside morphological data on sperm diameter and micropyle morphometrics, to test how sperm and egg structure have coevolved. In doing so, we test for an important, but often unconsidered, interaction on the pathway to fertilization.…”

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