Developmental Patterns and Cell Lineages of Vermiform Embryos in Dicyemid Mesozoans

12 Pages·2001·5.1 MB·English

by  Hidetaka Furuya

Reference: Biol. Bull. 201: 405-416. (December 2001)
Developmental Patterns and Cell Lineages of
Vermiform Embryos in Dicyemid Mesozoans
HIDETAKA FURUYA1. F. G. HOCHBERG2, AND KAZUHIKO TSUNEKI1
Department ofBiology, Graduate School ofScience, Osaka University, Toyonaka, Osaka 560-0043,
Japan: and 2Department ofInvertebrate Zoology. Santa Barbara Museum ofNatural History,
2559 Puesta del Sol Road. Santa Barbara, California 93105-2936
Abstract. Patterns ofcell division and cell lineages ofthe interesting groups of lower invertebrates. Each species is
vermiform embryos ofdicyemid mesozoans were studied in characterized by a fixed number of cells. The somatic
fourspecies belonging to fourgenera: Conocyemapolymor- cells therefore undergo a limited number of species-
plui, Dicyema apalachiensis, Microcyema vespa, and specific divisions during embryogenesis. The analysis of
Pseudicyemanakaoi. Duringearly development, the follow- embryonic cell lineages in dicyemids is intriguing, since
ing common features were apparent: ( 1 ) the first cell divi- it may provide clues towards an understanding of the
sion produces prospective cells that generate the anterior simplest patterns of cell differentiation in multicellular
peripheral region ofthe embryo; (2) the second cell division animals. A comparative study of cell lineage and devel-
produces prospective cells that generate the posterior pe- opmental processes among related species of dicyemids
ripheral region plus the internal cells of the embryo: (3) in is also relevant to advance our understanding of morpho-
the lineage of prospective internal cells, several divisions logical evolution in these simple animals.
ultimately result in cell death of one of the daughter cells. Dicyemids produce two distinct types of embryos: ver-
Early developmental processes are almost identical in the miform embryos from an asexual agamete and infusoriform
vermiform embryos of all four dicyemid genera. The cell embryos from fertilized eggs (Furuyaetal.. 1996). From the
lineages appear to be invariant among embryos and are standpoint of morphological evolution, the vermiform em-
highly conserved among species. Species-specific differ- bryo is the more pertinent target for study because its shape
ences appear during later stages of embryogenesis. The is similar to that of an adult. The cell lineage of vermiform
number of terminal divisions determines variations in pe- embryos has been fully documented in only two dicyemids,
ripheral cell numbers among genera and species. Thus, the Dicyema acitticephalum and D. japonicum (Furuya et al.,
numbers of peripheral cells are fixed and hence species- 1994). Among other species, cell lineages have been de-
specific. scribed only to a limited extent in Microcyema vespa and
Pseudicyema truncatiim (Lameere, 1919; McConnaughey,
Introduction
1938; Schartau, 1940: Nouvel. 1947: Bogomolov. 1970;
All members ofthe phylum Dicyemida are found in the Lapan and Morowitz. 1975). Details of cell lineage in the
renal sacs ofbenthic cephalopod molluscs (Nouvel, 1947: phylum as a whole remain to be determined.
McConnaughey. 1951; Hochberg. 19901. Their bodies In this paper we describe the pattern ofcell divisions and
consist of the smallest number of cells among multicel- cell lineages in the embryogenesis ofvermiform embryos in
lular animals (usually 10 to 40) and are organized in a dicyemids belonging to four genera: Conocyema polymor-
very simple fashion. Although recent studies have re- pha. Dicvema apalachiensis. Microcyema vespa. and
vealed that they might not be truly primitive animals Pseudicyema nakaoi. Our data reveal that cell lineages in
deserving the name ofmesozoans (Katayama et al., 1995: vermiform embryos are highly conserved among species;
Kobayashi et al.. 1999). they are still one of the most but species-characteristic features appear in the laterembry-
ogenesis, and these are related to morphological evolution
Received 14 May 2001; accepted 17 August 2001. and speciation.
405
406 H. FURUYA ET AL
Materials and Methods Micmcyenia vespa
Specimens of Cotwcyenta polymorpha van Beneden, Agamete diameter is about 6 /J,m. The first division is
1882, Dicyema apalachiensis Short, 1962, Microcyema meridional and unequal, producing two daughter cells. A
vespa van Beneden, 1882, and Pseudicyema tninciinim and B. Cell B becomes the mother cell of the peripheral
(Whitman, 1883) were examined in the collections of the cells ofthe embryo's head. The second division involves
Department of Invertebrate Zoology, Santa Barbara Mu- only cell B. This division is occasionally skipped. It is
seum of Natural History, Santa Barbara, California. Cono- extremely unequal, producing two daughter cells that are
cveina polymorpha, found in Octopus rulgaris, was col- quite different in size. The smaller of these two cells
lected by Henri Nouvel in the Mediterranean Sea (Nouvel, ultimately degenerates without contributing to embryo-
1947). Microcvema vt'.v/w and P. tninciitiini, found in Sepia genesis. The third division, involving cell A, is latitudinal
offcinalis, were also collected by Nouvel in the Mediterra- and equal, producing two daughter cells, 2A and 2a. Cell
nean Sea (Nouvel, 1947). Dic\eimi apalachiensis, found in 2A is the mother cell of peripheral cells in the tail, and
Octopusjoubini, was collected by Robert B. Short in the cell 2a is the prospective axial cell. In the 2a lineage,
Gulf of Mexico off Florida (Short, 1962). extremely unequal divisions occur at around the 5- and
Specimens of Pseudicveimi nukaoi Furuya, 1999, were 7-cell stages. The resultant much smaller daughter cells
prepared forthis study. A total of57 hostcuttlefishes. Sepia remain attached to the larger daughter cells until they
esculenta, were purchased in the western part of Japan. ultimately degenerate without contributing to embryo-
When dicyemids were detected in the kidney of the host genesis. The fourth division, involving cell 2B, is merid-
cuttlefish, small pieces of renal appendages with attached ional and equal, producing two daughter cells, 3B and 3B.
dicyemids were removed and smeared on slide glasses. The At this 4-cell stage, two pairs of cells, 2A-2a and 3B-3B.
smears were fixed immediately in Bouin's fluid for24h and are arranged crosswise with respect to one another. The
then stored in 70% ethyl alcohol. The fixed smears were furrow of the fourth division coincides with the plane of
stained in Ehrlich's hematoxylin and counterstained in eo- bilateral symmetry of the embryo. The pattern of division
sin. Stained smears were mounted using Entellan (Merck). and the cell lineage are the same for the descendants ofcell
Embryos within the axial cell ofparent nematogens were 3B and those of cell 3_B. The fifth division, involving cell
observed with the aid of a light microscope under an oil- 2A, is latitudinal and equal; resulting in the 5-cell embryo.
immersion objective at a magnification of 2000 diameters. The plane of cell division coincides with the plane of
Cells were identified by their position within the embryo, bilateral symmetry, and it separates the right cell 3A from
their size, and the intensity ofstain taken up by the nucleus the leftcell 3A. Thesecellsdonotdivide furtherbut become
and cytoplasm. By careful examination, we were able to the two peripheral cells of the tail region, known as the
identify each swollen nucleus that was about to divide and uropolar cells (Figs. 2a-c. 3a).
each metaphase figure in terms ofthe cell that was about to The pattern of cell division beyond the 5-cell stage
divide intotwodaughtercells. Each developingembryowas changes from spiral to bilateral. After the 5-cell stage,
sketched at three optical depths, and three-dimensional di- divisionsoccurnotoneby onebut in pairs, and the divisions
agrams were reconstructed from these sketches. Measure- become almost synchronized. Subsequent developmental
ments and drawings were made with the aid of an ocular stages thus proceed with odd numbers ofcells, yielding, for
micrometer and a drawing tube (Olympus U-DA), respec- example, a 7-cell embryo, and so on.
tively. Fully formed embryos consisted at most of23 cells, Thesixthdivision isextremelyunequal. Both the 3B and 3B
and special techniques such as injection of a tracer and cellsdivide, and they produce apairoflargecells andapairof
videoscopy were not required for determination of the cell much smallerdaughtercells. The smallercells degenerate and
lineage. Early divisions of the vermiform embryos exam- eventually disappear. At around the 5-cell stage, cell 2a, the
ined in this study were the same as those of Dicyema prospective axial cell, undergoes an extremely unequal divi-
(iciiticep/uiliini and D. japoniciim (see Furuya et /.. 1994). sion. The resultant smaller cell degenerates and eventually
The terminology of cells used by Furuya et al. (1994) was disappears. The seventh division is equal, and results in the
adopted in designating the cells in the present paper. 7-cell embryo. Cells 4B and 4B divide and produce two pairs
ofdaughtercells, 5B' and5B2 plus 5B1 and 5B2. respectively.
The future anterior-posterior axis of the embryo corresponds
Results almostexactly tothe5B'-3Aaxisatthe 7-cell stage. Aboutthe
7-cell stage, cell 3a, the prospective axial cell, undergoes an
In the Dicyemida. two adult forms, the nematogen and extremely unequal division. The resulting smallercells degen-
the rhombogen, develop asexually from an agamete (axo- erate and eventually disappear.
blast) through a vermiform embryo within the axial cell of After the seventh division, the order of division is not
parent nematogens (Fig. I). necessarily identical among developing embryos. The
DEVELOPMENT OF VERMIFORM EMBRYOS 407
AX
DV
AX"
PP
IPI^-LDV
UP
"I
..
PP
f g
V
DV
AG
*
Figure 1. Light micrographs of nematogens in four species of dicyemids. Scale bars represent 10 jim.
Abbreviations: AG, agamete: AX, axial cell; DC, degenerating cell; DP. diapolar cell; DV. developing
vermiform embryo; MP, metapolarcell; PP. parapolarcell; PR; propolarcell; S. syncytium; UP. uropolarcell;
V.vermiformembryo.Microcycimi n:\pu: (a)wholebodyofayoungindividual; (b)avermiformembryointhe
axialcellofthenematogen. Conocyemapolymorpha'. (c)wholebodyofanematogen;(d)developingvermiform
embryos in the axial cell of the nematogen. Dicyema upalachiensis: (e) whole body of the nematogen.
Pi>eudic\einanakaoi: (f)wholebodyofanematogen;(g)developingvermiformembryosin theaxial cellofthe
nematogen.
5B1 cell pair divides equally and produces two pairs of ther pair divides further, and they form the anterior part
daughter cells, 6B" and 6B12 plus 6B" and 6B12. The of the embryo (Fig. 2a, b). The 6B22 cell pair develops
5B2 cell pair divides equally and produces two pairs of into the parapolar cells, while the 6B", 6B12, and 6B2i
daughter cells, 6B21 and 6B22 plus 6B21 and 6B22. Nei- cell pairs eventually form a syncytium, which is more
408 H. FURLIYA ET AL.
a 6B,Bi:. 6B;;
. 5B"2_4B'2 S^-Bi1, 6A'i:CA12'
5B
5B'1: SB-' 5B:2 5B
Figure!. Thelate-stagevermiformembryosoffourspeciesofdicyemids.Scalebarrepresents 10^m.Cilia
are omitted. See text forexplanations ofcell division notations. Other abbreviations: AG. agamete: AX, axial
cell;DP.diapolarcell;MP.metapolarcell;PR,propolarcell;PP.parapolarcell;S.syncytium;UP.uropolarcell.
Microcvemavespa: (a)alate-stageembryo(sagittaloptical section) noteanagamete(5a~)inthecytoplasmof
an axial cell (5a'l; (b) a late-stage embryo (sagittal optical section); (c) formed embryo (sagittal optical
section) pairs of 6B", 6B'2. and 6B:i cells form a syncytium (S) that is more conspicuously stained with
hematoxylin. Conocyemapolymorpha; (d)alate-stageembryo(sagittaloptical section) noteanagamete (6a~)
incorporated in the cytoplasm ofan axial cell (6a'): (e) a late-stage embryo (lateral view); (f) formed embryo
(lateral view) pairsof4B" and5B21 cells form propolarcells (PR) that are moreconspicuously stained with
hematoxylin. Dicyema apalachiensis: (g) 13-cell stage note an anaphase figure of4B': cell and a metaphase
figure of4B12 cell; (h) 15-cell stage the 5B1" and 5B':i pairs form the propolarcells (PR), while the 5B"2
and 5B'22 pairs form another type ofpolar cell, the metapolar cell (MP); (I) formed embryo (lateral view)
propolarcells and metapolarcells are more conspicuously stained with hematoxylin. Pseudicyema nakaoi: (j)
22-cell stage note a metaphase figure in 4B12 cell the plane ofthis division, in contrast to the divisions of
4B|:pair(Fig. 2g),areobliquetotheanterior-posterioraxis,andastheresult,cellsofthepropolartieralternate
withcellsofthemetapolartier(seeFig. 2k, 1);(k)alate-stageembryo(lateralview);(I)formedembryo(lateral
view) propolarcells and metapolarcells are more conspicuously stained with hematoxylin.
conspicuously stained with hematoxylin than the other becomes an agamete (Fig. 2a). At this stage, cilia are first
cells (Fig. Ib). The two parapolar cells cover more than evident on the peripheral cells. Cilia on the external
half of the syncytium (Fig. 2c). As peripheral cells are surface of the syncytium are directed anteriorly and are
formed, the prospective axial cell, 4a, divides unequally. more densely distributed than on other peripheral cells.
The large anterior cell, 5a', undergoes no further divi- The fully formed embryo consists of three types of
sions and becomes the axial cell, while the smaller pos- cells: peripheral cells, one syncytial cell, and an axial
terior cell, 5a2, is incorporated into the axial cell and cell, which contains two agametes (Figs. Ib, 2c). The
DEVELOPMENT OF VERMIFORM EMBRYOS 409
i-3a i-4a f.
H2a
AG
410 H. FURUYA ET AL
Conocyema polymorpha 3a) is incorporated into the inside of the embryo, and the
3B1 pair divide and rearrange their descendants. At around
Agamete diameter is about 7 fxm. The first division is the 11-cell stage, the 3a cell undergoes an extremely un-
meridional and unequal, producing two daughter cells that equal division.
are slightly different in size. A and B. The larger cell B The 13-cell stage is achieved by equal divisions of cells
becomes the mother cell of the peripheral cells of the 4A2 and 4A2. The resultant 5A21 and 5A22 pairs undergo no
embryo's head (propolar and parapolar cells). The second further divisions and become diapolar cells and uropolar
division involves only the smaller cell A. This division is cells, respectively. Soon after these divisions, the 4B2 pair
latitudinal and equal, producing two daughter cells, 2A and divide equally into two pairs of daughter cells, 5B21 and
2a. Cell 2A is the mother cell of the peripheral cells ofthe 5B22 plus 5B21 and 5B22. The 5B21 and 5B22 pair undergo
posterior trunk and tail, and cell 2a is the prospective axial no further divisions and become the propolar cells and
cell (Fig. 3b). In the 2a lineage, extremely unequal divisions parapolar cells, respectively (Fig. 2e, f). At around the
occur at around the 5-, 1 1-. and 13-cell stages (Fig. 3b). and 13-cell stage, the prospective axial cell 4a undergoes an
the resultant much smallerdaughtercells remain attached to extremely unequal division.
the larger daughter cells until they ultimately degenerate At the final stage ofembryogenesis, the prospective axial
without contributing to embryogenesis. The third division, cell, 5a. divides unequally. The large anterior cell, 6a',
involving cell B, is meridional and equal, producing two undergoes no further divisions and becomes the axial cell,
daughter cells, 2B and 2B. At this 4-cell stage, two pairs of while the smallerposteriorcell, 6a2, is incorporated into the
cells, 2A-2aand 2B-2B, are arrangedcrosswise withrespect axial cell and becomes an agamete (Fig. 2d).
to one another. The furrow of the third division coincides The fully formed vermiform embryoofConocyemapoly-
with the plane of bilateral symmetry of the embryo. The iiiorpha consists of 14 peripheral cells and one axial cell,
pattern ofdivision and the cell lineage are the same for the which contains one or two agametes (Fig. 2f). The head
descendants of cell 2B and those of cell 2B (Fig. 3b). The peripheral cells are composed of four propolar cells and
fourth division, involving cell 2A. is meridional and equal, parapolar cells. The propolar cells have short, dense cilia
resulting in the 5-cell embryo. The plane of cell division and form the calotte, which is more conspicuously stained
again coincides with the plane ofbilateral symmetry, and it with hematoxylin than the other cells. Four diapolar cells
separates the right cell 3A from the left cell 3A. The pattern make up the trunk peripheral cells. The caudal peripheral
of cell division and the cell lineage are the same for de- cells are uropolar cells. The length, excluding cilia, of the
scendants of cell 3A and for those of cell 3A (Fig. 3b). fully formedembryo is about 25 /urn, and the width is about
The pattern of cell division beyond the 5-cell stage 10 jum. The cell lineage of the vermiform embryo is sum-
changes from spiral to bilateral. Beyond the 5-cell stage, marized in Figure 3b. No variations in cell lineage were
divisionsoccurnotone byone but in pairs,and theybecome found in more than 80 embryos examined.
almost synchronized. Subsequent developmental stages thus
proceed with odd numbers of cells.
At around the 5-cell stage, cell 2a, the prospective axial Dic\ciim (iptilcicliiensis
cell, undergoes anextremely unequal division. Theresultant
smallercell degenerates and finally disappears. The fifthcell Agamete diameter is about 5.5 jum. The first division is
division is equal and results in the 7-cell embryo. Thus, cells meridional and equal, producing two daughtercells ofequal
2B and 2B divide and produce two pairs of daughter cells, size. The subsequent patterns of development and cell lin-
3B' and 3B2 plus 3B1 and 3B2. respectively. The future eage up to the 7-cell stage are the same as those described
anterior-posterior axis of the embryo corresponds almost for CO/U>C\CIIHI polymorpha.
exactly to the 3B'-3A axis at the 7-cell stage. The sixth At the 7-cell stage, cells 3B2 and 3B2 undergo unequal
division is slightly unequal. Cells 3Aand 3A divide intotwo divisions, each generating a pair ofcells, one large and one
pairs of daughter cells, 4A1 and 4A2 plus 4A1 and 4A2. much smaller cell. The 9-cell stage is achieved by unequal
Cells 4A: and 4A2 are the smallest cells at this stage. In divisions of cells 3B1 and 3B1. The resultant small cells,
addition to the 2a lineage, cells 3B: and 3B2 undergo 4B'' and4B". divide again into two pairs ofdaughtercells,
unequal divisions, each generating a pair ofcells, one large 5B"' and 5B"2 plus 5B1" and 5B1'2. in the anteriorpart of
and one much smaller. The smaller cells degenerate and the embryo (Fig. 2g-i). Almost simultaneously, the resultant
finally disappear, although they remain in place on the large cells, 4B12 and 4B12, divide again into two pairs of
developing embryo until later stages. daughtercells, 5B121 and~5B122 plus 5B12' and 5B122. in the
The 3B1 pair divide equally into 4B11 and 4B12 pairs. anterior part ofthe embryo (Fig.m2h. i). These cells undergo
These cells undergo no further divisions. The 4B" pair no further divisions, and the 5B and 5B121 pairs become
become the propolar cells, and the 4B12 pair become the the propolar cells, while the 5B112 and 5B122 pairs become
parapolar cells (Figs. 2e, f). The cell in the 2a lineage (cell the metapolar cells. The cell in the 2a lineage (cell 4a) is
DEVELOPMENT OF VERMIFORM EMBRYOS 411
incorporated into the inside of the embryo, and the other to form the 19-cell-stage embryo. These pairs undergo no
cells,4B" and4B". divide andrearrange theirdescendants. furtherdivisions. Cell pair 5B2' become the parapolarcells,
At the 9-cell stage, cells 3A and 3A divide equally into and the 5B22 pair become the anterior diapolar cells (Fig.
two pairs of daughter cells, 4A1 and 4A2 plus 4A1 and 4A:. 3d). Cell pair 6A121 become the uropolar cells, and the
They do not divide further; the 4A' pair become the uropolar 6A122 pair become the posterior diapolar cells (Fig. 3d).
cells, and the 4A2 pair become the diapolar cells (Fig. 3c). At At the 19-cell stage, the 4B" and 4B12 pairs divide
the final stage ofembryogenesis, the prospective axial cell.4a. equally into two pairs of daughter cells, 5B111 and 5B"
divides unequally. The large anterior cell. 5a'. becomes the plus 5B121 and 5B122, in the anterior part of the embryo
axial cell, and the smaller posterior cell. 5a2, is incorporated (Fig. 2k. 1). Thesedivisions proceedcellby cell. The d"augh-
into the axial cell and becomes an agamete. ter cells undergo no further divisions, and the 5B1 and
The vermiform embryo of Dicyema apalachiensis con- 5B12' pairs become the propolar cells, while the 5B112 and
sistsof 14peripheral cells and one axial cell, whichcontains 5B122 pairs become the metapolarcells (Fig. 2j). The planes
one ortwo agametes. The peripheral cellsofthe head region of these divisions, in contrast to the previous division, are
are composed of four propolar cells, four metapolar cells, oblique to the anterior-posterior axis. As the result, cells of
and two parapolar cells. The propolar and metapolar cells the propolar tier alternate with cells of the metapolar tier
have short, dense cilia and form the calotte. Two diapolar (Fig. 2k, 1).
cells make up the trunk peripheral cells. Twocaudal periph- At the final stage ofembryogenesis, the prospective axial
eral cells are uropolar cells. The length, excluding cilia, of cell, 5a. divides unequally. The large anterior cell. 6a'.
the fully formed embryo is about 30 /nm. and the width is becomes the axial cell, and the smallerposteriorcell, 6a2, is
about 10 ij.m. The cell lineage of the vermiform embryo is incorporated into the axial cell and becomes an agamete.
summarized in Figure 3c. No variations in cell lineage were The fully formed vermiform embryo of P. nakaoi con-
found in more than 50 embryos examined. sists of22 peripheralcells and one axial cell, whichcontains
one agamete. The peripheral cells of the head region are
Pseudicyema nakuoi composed of four propolar cells, four metapolar cells, and
two parapolar cells. The propolar and metapolar cells have
Agamete diameter is about 6.5 /urn. The first division is dense short cilia and form the calotte. Ten diapolar cells
equal, producing two daughter cells of equal size. The make up the trunk peripheral cells. Two caudal peripheral
subsequent patterns of development and cell lineage up to cells are uropolarcells. The body length, excluding cilia, of
the 9-cell stage are the same as those described forDicvema thefully formedembryo isabout 70ju.m. andthebody width
apalachiensis (see Fig. 3c). is about 16 /am. The cell lineage of the vermiform embryo
At the 9-cell stage, cells 3B2 and 3B2 undergo extremely is summarized in Figure 3d. No variations in cell lineage
unequal divisions, each generating a pairofcells, one large were found in more than 50 embryos examined.
and one much smaller cell. The smaller cells resulting from
this division degenerate and finally disappear. In the 2a line, Discussion
unequal divisions occur at around the 5-. 9-. and 1 1-cell
stages (Fig. 3d). The pattern of development and cell lin- Patterns of development of the vermiform embryos of
eages up to the 9-cell stage are the same in both P. trunca- four species ofdicyemids belonging to four genera, namely
twn and P. nakaoi. Further development was not studied in Microcyema vespa, Coiwcveimi polymorpha, Dicvema
P. truncation, because adequate material was not available. apalachiensis, and Pseudicyema nakaoi, were studied in
Afterthe unequal divisions ofthe 3B2 pair, cell pairs 4A1 detail. In the embryogenesis ofeach species, cell divisions
and 3B1 undergo equal divisions almost simultaneously and proceed without variation and result in fully formed em-
produce two pairs of daughter cells. 5A11 and 5A12 plus bryos with a definite number and arrangement ofcells. The
4B" and 4B12. to form the 13-cell-stage embryo. At around process ofdevelopment ofvermiform embryos is very sim-
the 13-cell stage, the prospective axial cell.4a.undergoesan ple and seems to be programmed similarly to that of infu-
unequal cell division. The 5A11 pair divide equally and soriform embryos and infusorigens (Furuya el al.. 1992b,
produce two pairs of daughter cells, 6A1" and 6A112 plus 1993. 1995). Seven different cell lineages including those of
6A"' and 6A112. The plane ofthis division is parallel to the two previously described species, D. acuticephalum and D.
anterior-posterior axis, in contrast to the previous division, japonicum (Figs. 3.4; see also Furuyaelai, 1994), couldbe
which occurs parallel to the perpendicular axis. As a result, compared. Early developmental processes up to the 7-cell
cells 6A1" and 6A1" are situated on the left and right sides stage are almost identical in vermiform embryos examined
of the embryo, respectively. in this study and those ofD. acuticephalum and D. japoni-
At the 15-cell stage, cell pairs 4B2 and 5A12 undergo cum (Figs. 5. 6).
equal divisions almost simultaneously, and produce two Our results are different from earlier reports with respect
pairsofdaughtercells, 5B21 and 5B22 plus6A121 and6A122, to the timing of cell-fate specification (Lameere. 1919;
412
2a-r3a-r4a
AG
DEVELOPMENT OF VERMIFORM EMBRYOS 413
agamete (axoblast)
a-lineage
A-lineage
B-lineage
AG 2 3 4 5 7
9 11 13 15 17 19
Figure 5. Developmental processes ofvermiform embryos in several species ofdicyemids. The develop-
mental patterns and cell lineages from (he agamete (AG) to 7-cell stage are identical among the species. The
numeralsinthebottomrowrepresentcell numberstagesinthedevelopment. Arrowsinthedevelopingembryos
indicate daughtercells that were produced by the proceeding division, (a)Microcyema vespa', (b) Conocyema
polymorpha: (cl Dicyema apalachiensis', (d) D. acuticephalum with 16 peripheral cells; (e) D. acuticephaluin
with 18 peripheral cells; (fl D.japonicum; (g) Pseudicveina nakaoi.
Variations ofterminal divisions in cell lineages Furuya et ai, 1992a; 1994; Furuya, 1999). Such intraspe-
Species-specific patterns of development and cell lin- cific variation in peripheral cell numbers could be attributed
to minor differences in numbers of terminal divisions in
eages appear in the later stages ofembryogenesis. The most
certain cell lineages (compare Fig. 4a and b).
lstirniekaigneg tdhiaftfegrievneceriissesteoenvariinattieornmsinianlpdeirviipshieornaslicneltlhenuceml-l In the developmental patterns ofvermiform embryos, the
ebrearls.cFelolrninusmtbanecre,bseptewcieeesn-sDpeicciyfiecmadifafceurteincceesphianltuhme paenrdipDh.- oceclclurlibnielaagteesradlloy.noTthuvsa,ry,seavnerdaltheevteenrmniunmalbedrisvisoifopnesruispuhaerlally
japonicum can be attributed to the number of divisions of cells are formed as the result ofa pair ofterminal divisions
athdedi4tiAon1aplaitrer(mFiign.al4;diFvuirsuiyonasetocaciu,r1t9o9w4)a.rdInthoetheerndspoefcitehse, ivnarbioatblhetnheum2bAe-raonfdpeBr-icpehlelrallinceealgles,s.suIcnhsapsecDiiescytehamtahearvyeth-a
establishment of another cell lineage as well. The various rum, D. Ivcidoceum, and D. rhadinum, some peaks are
numbers ofterminal divisions, which are genetically deter- eFvuirdueynat, i1n99e9v).enThneumnbuemrbserofofpteerripmhienraalldicveillssio(nssemeatyabnloetsbien
mined, clearly play a significant role in the morphogenesis
of vermiform embryos and may be correlated with specia- strictly programmed in these exceptional species.
tion in the dicyemids.
Later development and lan'al morphologv
In most species ofdicyemids, vermiform embryos have a
constant number ofperipheral cells. However, some species In the evolution ofdicyemids, various types ofvermiform
ofdicyemids, such as Dicvema acuticephalum, D. hilohum, embryos must have been produced as deviations from a
D. benthoctopi, D. erythrum, D. lycidoceum, and D. rhadi- common developmental pattern. Unusual species, such as
nuni. have a variable number of peripheral cells (Nouvel, Microcyema vespa and Conocyema polymorpha, not only
1947; Couch and Short, 1964; Hochberg and Short, 1970; differ morphologically from otherdicyemids but are distinct
414 H. FURUYA ET AL
(axoblast) in M. vespa the developmental process may be truncated,
2a- {aagxaiamlectelel resulting in a simple cell lineage and a body organization
with a very small number of peripheral cells. However,
r3A peripheral cells oftrunk changes in cell lineage may not always contribute to mor-
] phological characters. For example, there are some differ-
L 1A .J (diapolars) & tail (uropolars) ences in the later cell lineage between Dicyema acuticeph-
AG tilitm and D. apalachiensis. but these dicyemids are very
similar in general body shape.
2BJ peripheral cells of head
1-5R- Cell death
B (propolars & metapolars)
McConnaughey (1951) described chromatin elimination
& trunk (diapolars) from the prospective axial cell. In Dicyema acuticephalum
[33BBJ-- and D.japonicum, what appears to be a mass ofeliminated
chromatin is actually a small cell that is produced as the
Figure 6. A common cell lineage in all the vermiform embryos ex- result of an extremely unequal division (Furuya et al.,
amined. At the first division, an agamete (AG) divides to produce two
daughtercells. A and B. Cell A divides intotwodaughtercells. Cell 2ais 1994). In Pseudicvenui tnincatnm and Microcyema vespa,
a mothercell for both an axial cell and agamete. Descendants ofcell 2A Lameere (1919) noted that the prospective axial cell under-
formtheperipheralcellsofbothtrunkandtail. DescendantsofcellBform went an unequal division and that the smaller daughter cell
the peripheral cells of both the head and anterior trunk. A cross (x) itself divided once or twice to produce two or four small
indicatesthatacell formed by unequal division degenerates anddoes not
contribute to the formation ofthe embryo.
Conocyerrndae- -Dicyemidae
in the later stages of development. As shown in the cla-
dogram (Fig. 7), these two species ofdicyemids are clearly
distinct and separate when compared with the clade com-
posed of the genera Dicyemu and Pseudicyema. Some
changes that occur in cell lineages certainly are reflected in
morphological features.
The genus Pseudicyema. as diagnosed by Nouvel (1933),
is morphologically very similar to Dicyema. As a result, it
occasionally has been treated as a subgenus of Dicyema
(Hochberg. 1990). The difference between Pseudicyema
and Dicvema depends on whether cells of the propolar tier
are alternate or opposite with respect to the cells in the
metapolartier. The developmental processes in these genera
are different only in the terminal cell lineage and the pattern
ofcell divisions at the final stage ofembryogenesis. On the
basis of cell lineages, differences between Dicyema and
Pseudicvema are within the range of inter-species differ-
ences in Dicvema, as shown in the cladogram. However, as Figure 7. Cladogram of six species of dicyemids based on cell lin-
eages ofthe vermiform embryos. These dicyemids might have been de-
far as calotte configuration and the process of calotte for- rived from an ancestorthathad abasiccell lineage as shown in Figure 6.
mation are concerned, Dicyemu and Pseudicyema can be Modifications in cell lineages might result in diversity of morphology
clearly identified as separate groups. Although cell lineage givingrisetotwoseparate families, namely.Conocyemidae and Dicyemi-
is an important character, it may not necessarily help to dae. Sketchesattopofcladogram indicatethesizeandshapeof thewhole
determine the definition of genera. Detailed comparative bdiofdfieersenotfcaedlulltlisnteaaggeessofaseafcohllsopwesc:ies(.1)BaErasrlryepdreesveenltopmomdeinfticaastiosnhsoowfnthien
studies on cell lineages and organization of infusoriform Figure 6. (2A) Calotte is formed with atierofpolarcells. (2B)Calotte is
embryos are also indispensable in separating dicyemid taxa. formedwithtwotiersofpolarcells:propolarsandmetapolarspresent.(3A)
In recent years, it has been argued that the evolution of Calotte form-, a syiicytium; diapolars absent. (3B) Calotte is cellular;
morphological features requires alterations in developmen- diapolars present. (4A) Calotte is formed from both 3B1- and 3B:-cell
tal processes. In dicyemids, the cell lineage ofMicrocyema larienelaogcesa.te(d4pBe)rCpaelnodtitceuliasrfloyrambeodveonmleytfaproolmar3sB.'-(c5eBl)llPirnoepaoglea.rs(5aAr)ePorbolpiqoulealrys
vespa isclosertoaconservative lineage than in othergenera oriented to metapolars. (6A) Cell death occurs both in 3B1- and 3B"-cell
in the phylum, but vermiform embryos ofM. vespa show a lineages. (oB) Cell death occurs only in 3B--cell lineages; both 4A'- and
distinctive form not seen in other genera. It is possible that 4A2-cells undergo no further divisions.