The partitioning of resources between male and female offspring is a fundamental aspect of the reproductive biology of animals. Sex allocation can be adjusted by controlling offspring sex ratio or by sex‐specific resource allocation towards individual eggs. The latter is well‐known in birds and reptiles, but less studied in other organisms, especially haplodiploids with females that can adjust offspring sex via fertilization control.
Here, we investigated whether egg size can affect sex allocation in a haplodiploid thrips, Pezothrips kellyanus. First, we established that larger eggs are more likely to be fertilized and become females than smaller eggs, which remain unfertilized and become males. This size‐dependent fertilization may be regulated by anatomical constraints that allow sperm access only to larger eggs. Furthermore, across both sexes, larger eggs developed into offspring with higher fitness, suggesting that egg size is a reliable proxy for resource allocation in this species.
During the early phase of reproduction, mating increased investment by females towards egg size thereby facilitating fertilization success and increased female offspring survival. However, this came with a reduced lifetime fecundity, reflecting that mating manipulates reproductive strategies in favour of increased daughter production. This may be in conflict with the optimal offspring sex ratio of females which is theoretically less female‐biased than for males.
We detected both egg size‐mediated sex allocation and a mating effect on resource allocation under both laboratory and field conditions, suggesting that they are maintained by natural selection.
We provide evidence for a finely tuned egg size‐dependent fertilization mechanism that mediates sex allocation strategies and is moderated by mating. Furthermore, our findings establish a new perspective about potential effects of sexual conflicts on egg size and sex allocation.
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In females of haplodiploid animals, female production requires fertilization, whereas male production does not. Therefore, haplodiploid species can display extraordinary sex ratios. Constrained sex allocation occurs when a female cannot produce daughters. This can be due to virginity but may also occur after mating due to insemination failure, selfish genetic elements or physiological constraints. Here, we investigated the mechanism underlying constrained sex allocation in Pezothrips kellyanus. In this species some mated females produce highly female‐biased broods, yet, for unknown reasons, others produce extremely male‐biased broods. Using crossing experiments controlled for maternally inherited endosymbionts we confirmed that constrained females were successfully inseminated. Furthermore, male‐biased offspring production was not paternally inherited, ruling out paternal sex ratio elements previously identified as sex ratio distorters in some parasitoid wasps. Next, we excluded mating time and paternal fitness effects (male size) on sex allocation. However, we found that constrained sex allocation only occurred in small females producing smaller eggs than large females producing larger eggs and female‐biased broods. Consequently, the bimodal sex allocation pattern correlates with maternal condition, and may have arisen (adaptively or non‐adaptively) in association with an egg size‐mediated fertilization mechanism recently detected in some haplodiploids, with egg size positively affecting fertilization success.
Heterostigmatic mites (Heterostigmata) are highly diverse, have symbiotic associations with various insects and occur worldwide. Despite Australia's rich insect biodiversity, little is known about its heterostigmatic mites. In this study, we conducted a preliminary study on heterostigmatic mites associated with beetles in eastern Australia and found four species, of which three were new: Caraboacarus stammeri Krczal, 1959 (Caraboacaridae) and Eutarsopolipus orpheus Katlav & Seeman, sp. nov., (Podapolipidae) associated with Australian‐endemic Gnathaphanus melbournensis (Castelnau, 1867) (Carabidae); Hoplocheylus australiensis Katlav & Seeman, sp. nov., (Tarsocheylidae) phoretic on Australian‐endemic Mastachilus quaestionis (Kuwert, 1891) (Passalidae); and Pavania gazellatris Katlav & Seeman, sp. nov., (Dolichocybidae) phoretic on introduced Digitonthophagus gazella (Fabricius, 1787) (Scarabaeidae). The species of Caraboacaridae, Dolichocybidae and Tarsocheylidae are the first species records of these families in Australia. It is likely that two of the three new species are Australian endemics, while P. gazellatris may be a more recent arrival on introduced dung beetles, possibly arriving with imports of exotic adult dung beetles. Furthermore, we provide an overview of the heterostigmatic mite fauna of Australia that consists of less than 90 species belonging to 13 families, with representatives of all known seven superfamilies. These mites have so far been mostly reported on beetles but also on other insect orders (Blattodea, Diptera, Hemiptera, Hymenoptera, Orthoptera, Thysanoptera). Our findings of three new species and one new record based on the analysis of a relatively small number of specimens and beetle species suggest that more comprehensive studies of host insect taxa will reveal a substantially larger Australian diversity of heterostigmatic mites.
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