Ovary architecture of two branchiobdellid species and Acanthobdella peledina (Annelida, Clitellata)

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• 作者：Piotr ?wi?tek^a ; ^{piotr.swiatek@us.edu.pl} ; Anna Z. Urbisz^a ; Witold Stru?y¨½ski^b ; Bartosz J. P?achno^c ; Aleksander Bielecki^d ; Stanis?aw Cios^e ; Erno Salonen^f ; Jerzy Klag^a
• 关键词：Ovary evolution ; Oogenesis ; Germ-cell cysts ; Cytophore ; Ultrastructure
• 刊名：Zoologischer Anzeiger - A Journal of Comparative Zoology
• 出版年：2012
• 出版时间：March 2012
• 年：2012
• 卷：251
• 期：1
• 页码：71-82
• 全文大小：2907 K

文摘

The aim of this study was to present data about ovary organization and oogenesis in two small groups of clitellate annelids, i.e. in representatives of Acanthobdellida (Acanthobdella peledina) and Branchiobdellida (Branchiobdella pentodonta and Branchiobdella parasitica), and to compare them to ovaries known from true leeches and oligochaetous clitellates. In A. peledina, the ovaries have the form of elongated cords, termed ovary cords, and are enveloped by coelomic sacs, the so-called ovisacs. The ovisacs are paired and each one contains only one ovary cord. The morphology and structure of the ovary cords depend on the maturity level of the animal. In young specimens the ovary cords are short and contain mainly oogonial cells and germ cells entering meiosis. Oogonia divide mitotically without full cytokineses, and as a result germ-line cysts are formed. As the animals grow, the cords become more elongated and the germ cells within the cords differentiate into nurse cells and oocytes. Oocytes gather cell organelles and, finally, detach from the ovary cord and float freely in the ovisac lumen.

In both examined branchiobdellidans the ovaries are also paired. They are short and conical and are not enclosed within ovisacs. The narrow end of each ovary is connected to the intersegmental septum via a ligament, whereas the outermost (broad) end of the ovary extends freely into the coelom. The ovaries are polarized. Their narrow ends contain oogonia, whereas nurse cells and growing oocytes, gradually projecting from the ovary, can be found in their middle and outermost parts. Early vitellogenic oocytes detach from the ovary and float freely in the coelom.

In all of the species studied, the ovaries are made up of germ-line cysts associated with somatic (follicular) cells. The architecture of a germ-line cyst is exactly the same as in other clitellate annelids that have been studied to date. Each germ cell in a cyst has one stable cytoplasmic bridge connecting it with a central anuclear cytoplasmic mass, a cytophore. The fate of germ cells constituting cysts is diverse. The majority of the cells withdraw from meiosis and become nurse cells; only a few continue meiosis, grow and become oocytes. The meroistic mode of oogenesis is suggested. We suggest also that the formation of germ-line cysts and ovary meroism should be regarded as basal conditions for all Clitellata. The occurrence of ovisacs enveloping the ovaries in A. peledina and Hirudinida is regarded as a synapomorphy of both groups, whereas ovaries found in B. pentodonta and B. parasitica have no ovisacs and resemble ovaries described in Oligochaeta sensu stricto.