Burmistrova Yu.A.1,2,3*, Kosevich I.A.1 2026. A comparison of oocyte development pathways in athecate and thecate hydroids // Invert. Zool. Vol.23. No.1: 1–24 [in English].
1 Department of Invertebrate Zoology, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
2 Laboratory of Marine Invertebrate Morphology and Ecology, A.N. Severtsov Institute of Ecology and Evolution RAS, Leninsky Prospekt 33, Moscow 119071 Russia.
3 Laboratory of Molecular Genetics, Russian Federal Research Institute of Fisheries and Oceanography, Okruzhnoy proezd 19, Moscow 105187 Russia.
* Corresponding author
Yuliya Burmistrova: yuliya.burmistrova@yandex.ru ORCID https://orcid.org/0000-0002-6976-1073
Igor Kosevich: ikosevich@gmail.com ORCID https://orcid.org/0000-0001-8604-2161
doi: 10.15298/invertzool.23.1.01
ABSTRACT: The differentiation of germ cells in most bilaterians occurs once during embryogenesis, while in primitive multicellular organisms gamete differentiation can occur continuously in their adult life. Germ cells in colonial hydroid polyps, for example, are believed to undergo continuous differentiation from interstitial stem cells (i-cells) during reproductive periods throughout their lifespan. However, the exact time and sites of germ cell differentiation remain an intriguing question in evolutionary research. In some colonial hydroids, the reproductive zooids develop only during the reproductive period. In others, medusae or gonophores (reduced medusae) develop directly on the feeding polyps (gastrozooids). For such species, the specific zone of gamete differentiation remains unknown. At the same time, the migration of gametes or their precursors along the coenosarc (the soft body of the colony) to the areas of reproductive zooid formation has been shown for some species of the thecate hydroids. A closely related question is the relationship between the processes of gamete differentiation and the initiation of reproductive structure development. Applying light microscopy, transmission electron microscopy and immunocytochemical methods, we followed the appearance and fate of the female germ cells in two colonial hydroid species. In thecate Gonothyraea loveni the growing oocytes migrate along the coenosarc of the shoots towards developing reproductive zooids. It has been experimentally confirmed that the initiation of the development of G. loveni reproductive zooids is independent from the presence of oocytes in their tissues. However, the presence of oocytes in reproductive zooid tissues is necessary for the completion of the normal development of these structures. The normally developing oocytes of athecate Clava multicornis are first identified in the gastrodermis of the hydranth body wall protrusions, which are the rudiments of gonophores. The abnormal oocytes for the first time were found in C. multicornis gastrozooid pedicels. Their investigation allowed to assume that the gamete precursors of C. multicornis migrate along the hydranth body to the sites of reproductive structure development, similar to the behavior of oocytes in G. loveni. Moreover, there is reason to assume that the differentiation and initial growth of C. multicornis oocytes can occur independently of the development of reproductive structures. While the reproductive structures development in C. multicornis depends on the presence of oocytes at the sites of their formation.
KEY WORDS: Clava multicornis, Gonothyraea loveni, germ cells, ectopic oocytes, oocyte migration, reproductive zooids.