Abstract
BackgroundTwo types of excretory systems, protonephridia and metanephridial systems are common among bilaterians. The homology of protonephridia of lophotrochozoan taxa has been widely accepted. In contrast, the homology of metanephridial systems – including coelomic cavities as functional units – among taxa as well as the homology between the two excretory systems is a matter of ongoing discussion. This particularly concerns the molluscan kidneys, which are mostly regarded as being derived convergently to the metanephridia of e.g. annelids because of different ontogenetic origin. A reinvestigation of nephrogenesis in polyplacophorans, which carry many primitive traits within molluscs, could shed light on these questions.ResultsThe metanephridial system of Lepidochitona corrugata develops rapidly in the early juvenile phase. It is formed from a coelomic anlage that soon achieves endothelial organization. The pericardium and heart are formed from the central portion of the anlage. The nephridial components are formed by outgrowth from lateral differentiations of the anlage. Simultaneously with formation of the heart, podocytes appear in the atrial wall of the pericardium. In addition, renopericardial ducts, kidneys and efferent nephroducts, all showing downstream ciliation towards the internal lumen, become differentiated (specimen length: 0.62 mm). Further development consists of elongation of the kidney and reinforcement of filtration and reabsorptive structures.ConclusionsDuring development and in fully formed condition the metanephridial system of Lepidochitona corrugata shares many detailed traits (cellular and overall organization) with the protonephridia of the same species. Accordingly, we suggest a serial homology of various cell types and between the two excretory systems and the organs as a whole. The formation of the metanephridial system varies significantly within Mollusca, thus the mode of formation cannot be used as a homology criterion. Because of similarities in overall organization, we conclude that the molluscan metanephridial system is homologous with that of the annelids not only at the cellular but also at the organ level.
Highlights
Two types of excretory systems, protonephridia and metanephridial systems are common among bilaterians
The development of the metanephridia and the central pericard/heart complex occurs almost simultaneously, the processes leading to the formation of individual components are interlocked
The metanephridial system of the polyplacophoran Lepidochitona corrugata is formed from a coelomic anlage that gives rise to the central pericard/heart complex
Summary
Two types of excretory systems, protonephridia and metanephridial systems are common among bilaterians. The homology of metanephridial systems – including coelomic cavities as functional units – among taxa as well as the homology between the two excretory systems is a matter of ongoing discussion This concerns the molluscan kidneys, which are mostly regarded as being derived convergently to the metanephridia of e.g. annelids because of different ontogenetic origin. The second question concerns the homology between metanephridial systems of different phyla: Here most authors favour homoplasy [4,7,8,9,10,11,12] This applies to the Mollusca, where a homology of their metanephridia with those of other lophotrochozoans, such as annelids, had been rejected mainly because of differences in development [12,13]
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