FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: ©1988 Springer. This manuscript is an author version with the final publication available at http://www.springerlink.com and may be cited as: Eckelbarger, K. J., & Larson, R. L. (1988). Ovarian morphology and oogenesis in Aurelia aurita (Scyphozoa: Semaeostomae): ultrastructural evidence of heterosynthetic yolk formation in a primitive metazoan. Marine Biology, 100, 103‐115. doi:10.1007/BF00392960 Marine Biology 100, 103-115 (1988) Marine ::~~:~:':: Biology © Springer-Verlag 1988 Ovarian morphology and oogenesis in Aurelia aurita (Scyphozoa: Semaeostomae): ultrastructural evidence of heterosynthetic yolk formation in a primitive metazoan K. J. Eckelbarger 1 and R. L. Larson 2 I Reproductive Biology Program, Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway, Fort Pierce, Florida 34946, USA 2 Gelatinous Zooplankton Program, Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway, Fort Pierce, Florida 34946, USA Abstract The present study was undertaken because information on ovarian morphology and oogenesis in the lower In Aurelia aurita, the ovaries arise as horseshoe-shaped Metazoa could lead to a better understanding of the evo­ evaginations of the gastrodermis in the floor of four in­ lution of the invertebrate ovary. Also, Boyer (1972), in a terradial gastric pouches. Germ-cell islands arise within en­ study of oogenesis in a polyclad turbellarian, suggested dodermally-derived gastrodermal cells. Oocytes grow and that the oocytes of primitive organisms could be expected gradually bulge into the mesoglea while maintaining to synthesize their own yolk (autosynthesis) since they of­ physical contact throughout vitellogenesis with specialized ten have simple ovaries lacking accessory cells or other po­ cells called trophocytes. Ultrastructural changes suggest tential sources of yolk proteins. Autosynthetic yolk forma­ that these cells transport yolk precursors from the coelen­ tion appears to predominate in the lower Metazoa, teron to the oocytes in a manner similar to that reported whereas the oocytes of many higher animals appear to de­ for the trophonema cells of anthozoan ovaries. Vitellogen­ rive their yolk from extraovarian sources (heterosynthesis) esis involves both the autosynthetic activity of the oocyte (Anderson 1974, Eckelbarger 1983). The oocytes ofAurelia organelles (Golgi complex and rough endoplasmic re­ aurita might be expected to utilize autosynthetic yolk for­ ticulum) and the heterosynthetic incorporation of pre­ mation because the animal is a primitive diploblastic cursors through endocytotic processes involving both coat­ metazoan with a low grade of tissue organization (Hyman ed pits and vesicles and smooth-surfaced tubules. Ul­ 1940). Kessel (1968) and Anderson (1974) provided sup­ trastructural data suggest that different types or classes of port for this assumption by describing autosynthetic yolk yolk precursors enter the oocyte through the trophocytes formation in the oocytes of an unidentified trachyline hy­ and via the surrounding mesoglea. To our knowledge, this drozoan medusa and A. aurita, respectively, although the is the most primitive animal in which this type ofyolk syn­ latter author did not provide ultrastructural details. In this thesis has been described. The trophocyte-oocyte relation­ ship in oocytes of A. aurita is reminiscent of the tro­ phonema-oocyte relationship in anthozoans and supports the belief that the Anthozoa and Scyphozoa share a close phylogenetic relationship. Introduction Aurelia aurita is a cosmopolitan scyphozoan common in all seas except the Arctic (Larson 1976). Studies on the life cycles of cnidarians began with A. aurita (Sars 1829) and the gross morphology of the ovary has been described (Sie­ ORAL ARM bold 1839, Claus 1883, Widersten 1965, Russell 1970). Most information on the morphology of A. aurita dates back to the nineteenth century, and ultrastructural studies have been confined to non-reproductive tissues (see Heeger and Fig. 1. Aurelia aurita. Diagrammatic radial section, showing loca­ Moller 1987). tion of ovary 104 K.J. Eckelbarger and R. L. Larson: Ovarian morphology and oogenesis in A. aurita K. J. Eckelbarger and R. L. Larson: Ovarian morphology and oogenesis in A. aurita 105 paper, we describe the ultrastructure of the ovary in canals radiate away from the distal perimeter of the A. aurita and present evidence that its oocytes utilize pouches. A thin membrane covers the subumbrellar sur­ heterosynthetic mechanisms of yolk formation during oo­ face of the pouch. A ring of thick subumbrellar mesoglea genesis. To our knowledge, this is the most primitive ani­ surrounds this membrane. The opening of the ring is mal in which this type ofyolk synthesis has been reported. referred to as the subgenital pit. The gastrodermis lining the floor of each gastric pouch evaginates to form a large, ribbon-like genital fold. Along Materials and methods the inner portion of the fold are numerous digitate pro­ cesses called gastric cirri. The gametes are distributed along Medusae of Aurelia aurita (10 to 15 em diam) were col­ the outer portion (Fig. I). The genital fold, within which lected in Nassau Harbor, Bahama Islands, in November the germ cells arise, is mostly flat with few secondary folds 1987. They arrived in the laboratory a day later and were in small medusae, but becomes more complex and convol­ maintained in filtered sea water at 20° to 23°C. The uted with sexual maturity (Fig. 3). The "ovary", is a meso­ gonads of the medusae were small and pale and there were glea-filled sac of gastrodermal cells into which developing few eggs in the oral arm brood pouches 48 h after col­ oocytes bulge as they develop (Fig. 39). The ovary is mostly lection. We began feeding the medusae newly hatched Ar­ free along its outer, abaxial margin, creating a sub genital temia sp. nauplii several times each day. After 4 d, the sinus between the ovary and the gastrodermis below. The ovaries were large and orange in color and large numbers subgenital sinus communicates with the coelenteron, and of eggs and developing embryos were present in the oral thus both surfaces of the gonad communicate with the gas­ arm brood-pouches. At this time, the gonads were removed trovascular cavity. and fixed for cytological study. For electron microscopy, pieces of the ovary were ex­ cised and fixed for I h at room temperature by emersion in Oogenesis 2.5% glutaraldehyde containing 0.2 M Millonig's phos­ phate buffer and 0.14 M sodium chloride. Tissue was then Oocytes arise from a germinal epithelium of endodermal rinsed for 15 min in three changes each of buffer wash origin consisting of islands of germ cells within surround­ (0.4 M Millonig's PO. mixed I: I with 0.6 M sodium ing gastrodermal cells. Three simultaneous stages of chloride) and postfixed for I h at room temperature in 1% oogenesis were observed in the ovaries of all specimens osmium tetroxide butTered in 0.1 M Millonig's phosphate examined: premiotic or spireme, previtellogenic and buffer and 0.38 M sodium chloride. Following fixation, tis­ vitellogenic. The majority of germ cells in these ovaries sue was rinsed in distilled water and dehydrated over a were early oocytes, but a variety of later stages were ran­ period of 2 h in ascending concentrations of ethanol, trans­ domly scattered throughout the gonad epithelium. No dis­ ferred through two changes of propylene oxide over a peri­ tinct population of mitotically-dividing oogonia was ob­ od of 10 min, and embedded in Epon. Thin-sections were served. The earliest oocytes are spherical cells in the zygo­ cut on a Porter-Blum MTl-B ultramicrotome with a dia­ tene/pachytene stage of meiosis with nuclei containing mond knife and stained for 10 min each with alcoholic synaptonemal complexes (Fig. 6). Most oocytes have nuclei saturated uranyl acetate and lead citrate and examined from 6 to 8,um in diameter containing a single nucleolus with a Zeiss EM 9-S2 transmission electron microscope. (Figs. 4, 7). Their cytoplasm is sparse, containing only free ribosomes, a few spherical mitochondria with tubular cris­ tae, and patches of fibrogranular material (nuage) of pre­ Results sumed nuclear origin (Fig. 8). A pair of centrioles, posi­ tioned at right angles to each other and associated with Ovarian morphology striated rootlets, are also common (Fig. 5). Previtellogenic oocytes, ranging in diameter from 15 to The sexes are separate in Aurelia aurita. The gonads lie in 20,um, begin cytoplasmic and nuclear growth resulting in the floor of interradial gastric pouches and appear clearly the formation of a prominent germinal vesicle. Oocytes at through the body wall as four horseshoe-shaped structures this stage protrude into the mesoglea, but still maintain (Figs. 1~3). The pouches communicate adaxially with the substantial contact with adjacent oogonia (Fig. 9). A thin mouth by a narrow tube, and a number of gastrovascular basal lamina covers the entire surface of the oocytes within Figs. 2-8. Aurelia aurita. 2: Lateral view, live female; arrowhead indicates ovary (one-half life size). 3: Subumbrellar view of single ovary showing folded nature of germinal epithelium and large numbers of oocytes (arrowed) (7 x). 4: Light-microscope 1 ,urn section through germinal epithelium showing numerous young oocytes (*); YO: vitellogenic oocyte (2 200 x). 5: Two centrioles (C) and associated striated rootlet (R) in cortical ooplasm of young oocyte (27 000 x). 6: Early ooycte with synaptonemal complexes (arrowed); N: nucleus, SGS: subgenital sinus (8700 x ), 7: Small oocyte with prominent nucleolus (Nu) within its germinal vesicle (N), adjacent to larger early vitellogenic oocyte (OC) on right; arrowheads indicate small yolk bodies forming in cortical ooplasm; M: mitochondrion; SG: subumbrellar gastro­ dermis, SGS: subgenital sinus, TR: trophocyte (5550 x); 8: Early oocyte with perinuclear aggregations offibrogranular material (nuage, *); M: mitochondrion; N: nucleus, Nu: nucleolus (10200 x) 106 K.J. Eckclbarger and R.