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Species of Monogononta produce at least three kinds of eggs, each with a different shell: amictic, male, and mictic (diapause). As coverings, eggshells function to protect the embryo from the external environment but may have other functions including to facilitate transport (i.e., hydrochory, zoochory, or anemochory). The fine structure of rotifer eggshells is poorly known despite their important functions. Here, we investigate rotifers from 10 families that live in different environments (marine and freshwater) and have varied lifestyles (planktonic and sessile). We use transmission electron microscopy to characterize eggshell layers and determine whether there are commonalities among eggshells within a species (amictic vs. mictic) and among related species and/or species from different environments and lifestyles. Our findings indicate that sessile species that live in protective tubes possess the thinnest amictic eggshells with minimal complexity. Planktonic species tend to have thicker amictic eggshells, but their thickness is not always related to having more shell layers. Resting eggshells tend to have the largest number of shell layers and are often the thickest shells. Many shells also possess spaces between layers that are interpreted as adaptations for floatation or anemochory.
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First ultrastructural description of an apomictic opsiblastic egg in freshwater Gastrotricha
Abstract Freshwater gastrotrichs have a biphasic lifecycle that reputedly involves the production of three types of eggs: apomictic and fast hatching (tachyblastic ova), apomictic and delayed hatching (opsiblastic ova), and plaque‐bearing eggs (potentially derived from mixis). While some details of oogenesis and eggshell structure are known for tachyblastic ova, there are few details on other egg types. Here, we provide the first ultrastructural description of the oviposited opsiblastic eggs of the freshwater gastrotrich,Lepidodermella squamata. Scanning electron microscopy revealed the eggshell surface to be ornamented with long flattened pillar‐like structures centered on polygonal plates that are pitted along their periphery. Transmission electron microscopy showed the pits to lead to a vast labyrinth of tubular spaces and larger cavities throughout the thick apical layer of the shell. The basal layer of the shell is amorphous and connected to a network of fine fibers that traverse an extra‐oocyte space and forms a protective sheet around the uncleaved oocyte. The uncleaved oocyte has a dense layer of peripheral ooplasm surrounding a core of organelles including mitochondria, membrane‐bound secretion granules, endoplasmic reticulum, and a single nucleus in a granular, ribosome‐rich cytoplasm. Secretion granules are the most abundant organelles and presumably contain lipid‐rich yolk that will be used as energy for delayed cleavage, thus functioning in temporal dispersal. These data are compared to the fine structure of invertebrate resting eggs across the phylogenetic spectrum to determine the novelty of opsiblastic egg structure inL. squamata.
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- Award ID(s):
- 2051684
- PAR ID:
- 10494574
- Publisher / Repository:
- Journal of Morphology
- Date Published:
- Journal Name:
- Journal of Morphology
- Volume:
- 285
- Issue:
- 1
- ISSN:
- 0362-2525
- Subject(s) / Keyword(s):
- development history, meiofauna parthenogenesis reproduction rotifer
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Diapausing embryos of invertebrates represent investments in future populations. Thus, these embryos must be capable of withstanding a variety of environmental assaults. Consequently, their eggshells should be adapted to resist injuries from predators, sediments, or excessive shrinkage if desiccated. To date, there have been no direct nanomechanical measurements of the eggshells of most diapausing invertebrates. Here, we used three approaches to understand how eggshells of two rotifers, a freshwater species (Brachionus calyciflorus) and a brackish water species (B. plicatilis), tolerate harsh conditions: (1) atomic force microscopy to measure elasticity and hardness; (2) transmission electron microscopy to study ultrastructure; (3) scanning electron microscopy to examine surface features. We compare these values to measurements of brine shrimp (Artemia salina) cysts and mosquito (Aedes aegypti) overwintering eggs. Our results revealed that rotifer eggshells are structurally similar and have comparable nanomechanical values. While rotifer eggshells had lower Young’s moduli (ca. 13–16 MPa) and hardness values (1.84–1.85x10-2 GPa) than eggshells of Artemia and Aedes, eggshells of all species were relatively elastic and not particularly resistant to deformation. Pliancy of shells that form egg banks (i.e., Artemia, Brachionus) may be an adaptation to resist cracking under the physical forces of buried sediments. Though there are no obvious relationship between eggshell thickness, ultrastructure, ornamentation, or nanomechanical values in rotifer eggshells, we hypothesize that eggshell chemistry may play an important role in determining elasticity and hardness. Future studies should consider an integrative approach to understand importance of eggshell structure, chemistry, and mechanics in protecting diapausing embryos.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/jmor.21659
