LIFE HISTORIES OF TWO SPECIES OF ASPIDOGASTRIDS AND THE PHYLOGENY OF THE GROUP

Trematodes belonging to the subclass Aspidogastrea are of extreme phylogenetio in-terest. They were regarded to be a group of trematodes intermediate in systematic posi-tion between Digenea and Monogenea, After its establishment in 1936, it had beenwidely accepted by helminthologists (Dawes, 1941, Dollfus, 1953; Skrjabin, 1952;Chauhan, 1954; Rohde, 1971, 1973). However, knowledge regarding their biology andepizootics is not much. Life cycles are only known for a few of them such as Aspido-gaster conchicola K. Baer (See Aubert, 1855; Voeltzkow, 1888; Williams, 1842), As-pidogaster indica Dayal (See Rai, 1964), Multicotyle purvisi Dawes (See Rohde, 1971)and Lobatostoma manteri Rohde (See Rohde, 1973). More recent approach to the problemis to study them as an ancient group of trematodes probably occupying a phylogeneticposition "close to the root of Digenea" as Rohde (1973) has expressed it. For more than thirty years we have been collecting specimens and accumulating dataon this interesting branch of flatworms with a purpose to study it as a model of pre-exi-sting group of digenetic trematodes so as to understand their origin and evolution. Alto-gether five species of Aspidogastrids were collected by us from Fujian (Fukien) pro-vince. In addition there were recorded the occurrences of Aspidogaster amurensisAchmerov, 1956 in Heilungjiang and in Hubei province. The five species occurring inFujian are as follows: Aspidogaster conchicola K. Baer Aspidogaster indica J. Dayal Aspidogaster ijimai Kawamura Cotylaspis sinensis Faust and Tang Lophotaspis orientalis Faust and Tang Of these A. ijimai Kawamura and A. in lica Dayal are reported for the first time inChina. Life cycles of two species, Lophotaspis orientalis and Aspidogaster indica, wereelucidated. For the former the molluscan host is Corbicula fluminca Muller and verteb-rate host is Amyda tuberculata (Cantor) (=Trionyx sinensis Weigmann) and for thelatter the molluscan host is Limnoperna lacustris (v. Martens) and the vertebrate hostsare two species of fresh-water fishes, Megalobrama terminalis (Rich.) and Squaliobar-bus curriculus (Rich.). Observations were made on the gametogenesis of Aspidogaster indica. The pheno-menon of maturation and fertilization were studied. The primary spermatogonia occuron the periphery of the testis. They appear as masses of cells beneath the wall and arecomparativety small in size. They divide by ordinary mitotic divisions to form the se- condary and tertiary spermatogonia. The primary spermatocytes are formed by thedivision of the tertiary spermatogonia and often appear in a cluster of cells connectedtogether by cytoplasmic strands. Meiotic division occurs in primary spermatocytes toform secondary spermatocytes, which also appear as syncytial rosettes. The secondaryspermatocytes, by mitotic division, give rise to spermatids, the nuclei of which elongateand gradually rupture the cytoplasm and extend beyond the cell body. After nuclearelongation they transform to be spermatozoa which are filamentous with swollen heads.The spermatozoa slough off from the cytoplasmic residue and gather together in bundlesfilling diffusely in different portions of the testis. Oogonia occur in the anteriormost of the ovary. They form a layer of small cellssurrounding the periphery of the organ. These cells divide in ordinary division andbecome oocytes which are much larger than the oogonia. They are usually found in theinterphase condition. The chromatin materials spread over the nucleus irregularly likeblack reticulum with a round nucleolus placed eccentrically. Maturation prophase ofoocyte does proceed beyond pachytene, while the oocyte is still in the ovary. As itenters the proximal portion of the uterus, the penetration of the spermatozoon occurs.The penetrated spermatozoon appears as a small irregular mass of chromatin materialembedded in the ooplasm. During maturation division the first change in the nucleusis the formation of chromosome filaments, which gather to the central part of the cell,the nuclear membrane having disappeared. In prophase the chromosomes which firstappear in pachytene now contract into diplotene-bivalents and appear in various shapessuch as rings, crosses, single or double twisting loops etc. The tetrads are in this condi-tion at the end of the prophase, and then become more and more compact until thetypical solid forms appearing in the meiotic metaphase. With the formation of thefirst polar body, chromosomes, which are now in diad, appear more diminutive and arestained more faintly. The maturation follows the first without interkinesis. Since itis rapidly completed, no clear chromosome counts are possible. The first and secondpolar bodies were observed in our squashes to one side of the egg cell or may detach andscattered themselves among the yolk nuclei. It was determined that the haploid numberis 6, but some oocytes show 7 or 5. Such variations in number may be due to faultytechnique, or to the natural phenomenon of chromosome behavior in trematodes, asreported by other investigators in trematode cytology. After the formation of male andfemale pronuclei, the fertilized ovum soon enters into fhe first cleavage division. Thetwo pronuclei are resolved into two separate groups of chromosomes. Of the two suchgenomes the male chromosomes are much smaller. Their fusion gives birth to a zygote,the first cell of the next generation. In the first cleavage division the zygote first shows chromosomes in thin filamentsdistributed in the cell at random. They pass into pachytene and diplotene stages andthen arrange themselves on the equatorial zone. Subsequently the cell divides unequallyinto one large and one small blastomeres. The second cleavage division occurs in thelarger blastomere resulting in three cells of nearly the same size. Then one of the newlyformed blastomere divides unequal1y into one large and one small cell. The latter is thefirst formed micromere. The next stage observed is an embryo with 7 cells of differentsizes, consisting of 3 macromeres, 2 mesomeres and 2 micromeres. In the embryo of11 cells there are one macromere, 6 mesomeres and 4 micromeres. The embryo with 19 cells possesses its cell elements much closer in size, with larger ones about as large asmesomeres. Such pattern of embryonic development indicates that its components un-dergo cell divisions not in synchrony but at different rates, some being more rapidlywhile others lagging behind. But gradually they turn to be of the same size. Although gametogenesis has been extensively studied in Digenetic trematodes, nonehas been made in Aspidogastrids. Gresson (1964) has made a review, in which are listed60 species of Digenea the gametogenesis of which has been described. They belong to54 genera and 23 families. Monogenetic trematodes, such as Gyrodactylus elegans and Polystoma integerrimum were studied with regard to gametogensis and development inthe early part of the present century (Kathariner, 1904; Goldschmidt, 1902; Gille,1914). It can be seen that the basic procedures of fertilization and embryonic development are fundamentally the same in these groups of helminth parasites. The development of Lophotaspis orientalis was further traced in its intramolluscanphase. While the direct development of this group of trematodes has been universallyaccepted, our observation on its successive growing stages gave actual proof for this conception. The different stages marked A-H (fig. 2, 4, 5; fig. 3, 1-5) were tabulated withmeasurements. Special emphasis was made on the number of alveoli in the sucking diskof each stage. The sucking disk is a well developed adhesive organ adapted for the locomotion on the tissue surface of its molluscan host. As shown by its genealogy, thiscomplicated organ was derived from a simple posterior sucker. Discussions were made on the phylogeny and taxonomic relationships of Aspido-gastrea to other groups of helminthes. The cotylocidia of both species were studied andcompared with the miracidia of the Digeneans, oncomiracidia of the Monogeneans andthe larvae of the Temnocephala species. The archaic characters of the larval structureare represented by the rhabdocoelic digestive gut, the large doliiform type of pharynxand pigmented eyespots etc. With regard to the adult structure and habitat the close resemblance of Aspidogastrea to Digenea impressed some eminent helminthologists likeLeuckart and Braun. Leuckart (1879) expressed the idea that the Aspidogastrids areessentially mature rediae. Recently Stunkard (1962; 1963) affirmed that the affinitiesof the Aspidogastrids are clearly with the Digenea. As believed by many helmintho-logists, the parasitic flatworms, especlally trematodes might have arisen from dalyellioidrhabdocoels, to which the Aspidogastrids are much alike. It is therefore, highly reward-ing to consider the Aspidogastrids as the forerunner of the present-day Digenea, themodel of an ancient group, the hypothetical Prodigenea. Needless to say, in contrast to alternation of generations, the direct development ofAspidogastrea is an important distinguishing feature separating it from Digenea. WithMonogenea it differs in several important aspects: the absence of posterior haptor andchitinous hooks, the posterior position of excretory pores, etx. It seems, therefore, anindependent status of Aspidogastrea is justified.