Tải bản đầy đủ - 0trang
Chapter 24. Cytogenetic studies on marine myodocopid Ostracoda: The karyotypes of Gigantocypris dracontovalis Cannon, 1940 and Macrocypridina castanea (Brady, 1897)
Cytogenetic Studies on Marine Myodocopid Ostracoda 295
the Cypridinacea within the Myodocopina. It is probably cosmopolitan in its distribution (Angel,
1981). The material on which this work is based was recovered from samples taken in the N.E.
Atlantic. Although the majority of the specimens were obtained from hauls taken between 3,640
and 3,465 m, a few others occurred in near bottom samples at 4,031 m. G. drucontovulis is one of
the largest living ostracod species (only slightly smaller than G. muelleri). Females are larger than
males. The size of N.E. Atlantic specimens ranges between 8-10 mm for the female and 7-9 mm
for the male (A.M. in prep.).
Mucrocypridinu custuneu (Brady, 1897) which also belongs to the Cypridinacea is a species of
cosmopolitan distribution. Its adults are deep mesopelagic in habit. The specimens studied in this
paper were recovered from hauls taken between 1,530 and 350 m. M . custanea is also a large ostracod, although smaller than Gigantocypris. The size range of the female is between 4.6-7.6 mm and
for males, 4.8-6.4 mm.
As is the case with many cypridinaceans, the females of both G. dracontovulis and M . custuneu
retain the eggs in a brood chamber where they develop before being released into the water as free
Embryos as well as testes of both species were used for analysis of mitotic and meiotic stages
(Pl. 1; P1. 2, figs. la-e). These tissues proved to give the best results for the study of karyotypes, as already demonstrated in the study of G. muelleri (Moguilevsky, 1985). Due probably to the
great depths at which G. drucontovulis lives, added to the extremely bad weather conditions experienced during ‘Cruise 148’, none of the specimens were recovered alive. Therefore, all specimens
were fixed and preserved immediately in a 3:l absolute alcohol/acetic acid solution.
To best study chromosomes, one must encounter cells at the right stage of division. The method
employed is based on the blocking effect on mitotic divisions obtained when Colchicine is added
to the water of the medium in which the ostracods live. This produces an accumulation of cells at
metaphase stage, thus rendering the study of the number and morphology of the chromosomes easier. This Colchicine treatment was carried out on the few live specimens of M . custanea recovered.
The fact that no specimens of G. drucontovulis were found alive precluded their being cultured in
this way. Fortunately, however, enough cells were found to provide sufficient information on the
number and morphology of their chromosomes. All photographs were taken by one of us (A.M.)
using a Leitz Laborlux 12 compound microscope with a Wild Photoautomat MPS 45 camera
attachment. It was found that the best results were obtained by using a Kodak Image capture
ZAHU microfilm 5460.
The study of cells from testes and young embryos of G. drucontovulis reveals a karyotype composed of a diploid number of 18 for the female and 17 for the male. The complement is made up of
16 autosomes and 2 X chromosomes in the female and 16 autosomes and one unpaired X chromosome in the male (XO).
Although the karyotype of G. drucontovulisappears to be highly symmetrical and uniform, two
distinct groups of chromosomes can be distinguished. The first group is composed of three pairs
PLATE1-Figs. la-f. Gigantocypris dracontovalis, Mitosis in an embryo squash.
la. Low power field of view showing: Interphase (I); Prophase (P); Metaphase (M); Anaphase (A) and Telophase
(T) stages. lb. Prophase and interphase. lc. Metaphase. Id. Early anaphase. le. Late anaphase. If. Anaphase
Figs. 2a-f. Gigantocypris dracontovalis, Meiosis in testis tissue. 2a. Zygotene. 2b. Diplotene, showing 8 bivalents
and a single unpaired x(arrowed). 2c. Diakinesis (Metaphase I); single unpaired x(arrowed). 2d. Anaphase
I. Be. Prophase 11. 2f. Anaphase 11.
Cytogenetic Studies on Marine Myodocopid Ostracoda 297
of very similarly sized submetacentric chromosomes. The second group is made up of 6 pairs of metacentric or very slightly submetacentric chromosomes which are fractionally smaller than those of
the first group. Between the largest and the smallest chromosomes there is a very gradual size
gradient thus rendering difficult the identification of individual chromosomes exclusively on the
basis of size (Pl. 2, fig. 2b).
The same type of tissue was used to study the karyotype of M. custuneu. This shows a similar
complement to that of G. dracontovulis; 18 chromosomesfor the female (16A+XX) and 17 for the
male 16A+XO). Although the number of chromosomes is the same in both species, their morphology is very different.
Taking into account their size and morphology, the chromosomes of M. castaneu can be divided
into three clear groups. The first group, composed of the largest chromosomes, includes 8 which
are submetacentric, 4 of which show a secondary constriction in their long arm. The second group
includes 8 metacentric chromosomes, smaller than the submetacentric ones. The remaining 2
chromosomes (3rd group) are smaller than all the others and clearly acrocentric. Their size ranges
between 9 and 19 pm (Pl. 2, fig. 2c). The original study of G. muelleri revealed a karyotype comprising 18 chromosomes for the female (16+XX) and 17 for the male (16A+XO), all being metacentric (Moguilevsky, 1985).
Three species of cypridinacean Myodocopida have been studied to date. These are: Gigantocypris muelleri, a bathypelagic species which lives between 700-1 500 m depth in the N.E. Atlantic (Moguilevsky and Gooday, 1977); G. dracontovulis, a deep bathypelagic species which occurs at much
deeper levels (around 400 m) with no overlap in its vertical distribution with G. muelleri, and Mucrocypridinu custuneu which is meso- to bathypelagic in its vertical distribution.
All three species share the same number of chromosomes, their complement being 18 chromosomes for the female (16A+XX) and 17 for the male (16A+XO). The results of the research carried
out so far on these 3 species indicate some clear similarities and differences between their karyotypes. These are outlined below:
The karyotype of G. muelleri is highly symmetrical, presenting a great uniformity in both the
morphology and size of the chromosomes. All chromosomes are metacentric and their size ranges
between 19 and 24pm. The size gradient between the largest and smallest chromosomes is so
gradual as to render difficult the identification of individual chromosomes (Pl. 2, fig. 2a).
G. drucontovulis presents 6 chromosomes which are clearly submetacentric and the remaining
12 are metacentric or very sightly submetacentric and fractionally smaller than the others, Their
size varies gradually between 16 and 22pm (Pl. 2, fig. 2b).
Macrocypridinu custuneu presents 8 chromosomes which are both submetacentric and the
1argest;lfour of these 8 show a secondary constriction in their long arm. Of the remaining 10 chromosomes, 8 are metacentric and 2 clearly acrocentric. Their size ranges between 9 and 19 pm (Pl.
2, fig. 2c).
Although the number of chromosomes is the same, there are some differences in morphology
PLATE2-Figs. la-e. Macrocypridina castanea. la. Prophase (C-mitosis in embryo squash). 1b. Metaphase
(C-mitosis in embryo squash). lc. Diakinesis (Meiosis in ovary tissue). Id. Anaphase I1 (Meiosis in testis
tissue). le. Sperms (testis).
Fig. 2a. Gigantocypris muelleri; karyotype (female). Fig. 2b. Gigantocypris dracontovulis; karyotype
(fenqgle). Fig. 2c. Mucrocypridina castaneu; karyotype (female).
298 A. MOGIIJLEVSKY
AM) R. C. WHATLEY
and size. The two species of Giguntocypris are more similar to each other than either is to M . custuneu due to the presence of acrocentric elements in the latter species. When comparing the overall
appearance of chromosomes at metaphase stage, those of the first two species are similar and
‘baton’-like whereas those of M . custuneu appear stouter, with some of them resembling a “bowtie”. (Pl. 2, figs. 2a-c).
As shown in Table 1, the largest chromosomesare found in G. muelleri followed by G. drucontovulis; the smallest chromosomes occur in M.custuneu. This is in exact relation to the overall size
of the 3 species. The significance of their relationship is not as yet clearly understood.
Hinegardner (1976) reports a positive correlation between adult body size and DNA content
in certain animal species. Further study on both the karyotypes and DNA contents of many more
species of Ostracoda is required in order to test to what extent this theory applies to this group.
TABLE1 P O M P A R I S O N OF THE DIPLOID
COMPLEMENT, SIZE RANGE AND MORPHOLOGY
Diploid Chromosome Numbers
Comparison with the Podocopina
Tetart (1978) analysed the karyotypes of 24 species of freshwater podocopid Ostracoda. Of
these, 22 belong to the Cypridacea, 1 to the Cytheracea and 1 to the Darwinulacea. He found that
the differences between the karyotype of the Cytheracea and that of the Cypridacea are mainly
numerical, whereas the Darwinulacea differ also in the morphology of their chromosomes. Durwinulu stevensoni Brady and Robertson (1870) has a peculiar karyotype composed of 22 chromosomes all of which are ‘acrocentric’and very uniform in size. As can be seen in Table2, themorphology of the chromosomes of the cyprids varies. Tetart suggests that the more ‘primitive’ karyotypes
are composed of only ‘acrocentric’chromosomeswhile more evolved karyotypes have less ‘acrocentric’ and more metacentric chromosomes.
Although the number of chromosomes of the Myodocopina studied to date seems to fall within
the range of most podocopid species, their morphology and size are very different since the majority (or all) are of a metacentric of submetacentric type. The chromosomes of the former are also
very much larger.
2-DISnUeVnON OP TYPES, DIPLOID
OF cHROMosoMEs WITHIN THE P O m P I N A
ON DATA TAKEN FROM TETART
Diploid Chromosome Numbers
[Limmcythere (L.) inopinata]
bisexual + parthenogenetic
Cytogenetic Studies on Marine Myodocopid Ostracoda 299
The karyotypes of the 3 species of Myodocopina studied to date revealed some clear features
which set the group apart from the Podocopina. Following Tetart’s reasoning, the Myodocopina
may prove to be a highly specialized group with a highly developed karyotype. However, other
species of Cypridinacea as well as Halocypridacea need to be studied before any firm conclusions
can be drawn regarding the position of the Myodocopina in the evolutionary plexus of the Ostracoda.
Some 15 species of Halocypridacea have been analysed to date (A.M. in prep.); unfortunately,
the results have been somewhat unsatisfactory possibly due to the inadequacy of the material. Some
modifications to the methods of culturing and fixing specimens of this particular group have already
been introduced. The results, which will be published in due course, might throw some light onto
the status of Halocypridacea within the Myodocopida and its comparison with the Podocopida.
Potential implication of future studies in the cytotaxonomy of Ostracoda
As explained above, Tetart in his work on freshwater podocopids suggested that evolution of
the karyotype in ostracods involves a reduction in the number of chromosomes with centromere
in a distal position which fuse to form chromosomes with the centromere in a median position or
metacentric. These Robertsonian fusions are found to be one of the processes by which a reduction in the number of chromosomes occurs in closely related species in the animal kingdom.
The evolution of the karyotype of many species has been studied using chromosome banding
techniques. One of the most important applications of chromosome banding is the unequivocal identification of the chromosomes in a karyotype, especially when these are of very similar size and
shape, as well as small parts of the chromosomes which may have undergone rearrangements. In
many cases the karyotypes of related species are remarkably similar. As a direct result of chromosome banding, karyotype changes have been shown to arise by several mechanisms such as Robertsonian fusion. Protocols for banding involve the pre-treatment of the chromosomes in such a way
as to cause their structure to collapse. During subsequent staining certain regions of the chromosome reconstitute to produce darkly staining bands.
With a view to testing Tetart’s ideas on the evolution of ostracod karyotypes, three different
chromosome banding procedures were tried on the species of Myodocopina studied herein. These
were, C-banding after barium hydroxide; Acetic acid-Saline-Giemsa (ASG) banding and Rbanding by high temperature treatment, as described by Macgregor and Varley (1983).
To the authors’ knowledge, these techniques have not been previously applied to Ostracoda.
Chromosome banding was first developed for mammalian species, and although the results on
Myodocopina are so far somewhat disappointing, the potential is very promising. These not very
satisfactory results are due mainly to the overlapping of the large chromosomesand a certain inadequacy of the techniques. Modification of these protocols is expected to produce better results in the
Conventional taxonomic studies on Ostracoda are based on the analysis of a wide spectrum of
characters of both carapace and appendages. Similarly, cytotaxonomy covers all aspects of taxonomy’at the cellular level. Cytotaxonomic studies deal not only with the number of chromosomes,
but also with their morphology and behaviour. Cytochemical information is now also being used.
The study of genetic variability by means of gel electrophoresis provides the opportunity for
understanding the interrelations between genetics and ecology.
The study of evolution at the DNA level entails the study of DNA gain and loss as well as of
nucleotide changes. Hinegardner (1976) suggests that “animals and plants that are considered to
represent primitive or ancient and relatively slowly evolving lineages often have more or much
more DNA than the average of their particular taxon”.
The authors are convinced of the potential importance of cytotaxonomic studies in Ostracoda.
300 A. MWUILEVSKY
AND R. C. WHATLEY
The analysis of the karyotypes of all major groups is projected and in the near future it is also
intended to begin a study of DNA amounts and gel electrophoresis of enzymes.
The overall results obtained to date are encouraging enough to consider the cytogeneticalcharacteristics of the Ostracoda as a useful tool in the understanding of their evolution.
A. Moguilevsky expresses her grateful thanks to the British Micropalaeontological Society
whose grant in aid made it possible to participate in the RRS Discovery Cruise 148 to the N.E.
Atlantic. Dr. Howard Roe, Principal Scientist, is thanked for his kindness and consideration during the cruise. Dr. Martin Angel is gratefully acknowledged for his constant encouragement and
help, and in particular, for the identification of halocyprid species. Dr. Celia Ellis also kindly
afforded considerable help in this respect during the cruise. The authors have relied very heavily
on both the generosity and expertise of Dr. Neil Jones, of the Dept. of Agricultural Botany, U.C.W.,
Aberystwyth; we thank him for his constant assistance and encouragement. Mrs. Marian Mayes is
thanked for typing the manuscript.
ANGEL, M.V.1981. Ostracoda. I n BOLTOVSKOY, D. (ed.). Atlas del Zooplancton del Atlfntico Sudoccidental, 543-585.
HINEQARDNER, R. 1976. Evolution of genome size. I n AYALA, F. (ed.). Molecular Evolution, Chapter 11: 179-199.
Sinauer Ass., Sunderland, Massachusetts.
MACGREGOR, H.C.and VARLEY, J.M.1983. Working with animalchromosomes.250 pp. John Wiley and Sons, New York.
MOGUILEVSKY, A. 1985. Cytogenetic studies on marme ostracods: the karyotype of Gigantocyprismuelleri Skogsberg,
1920 (Ostracoda, Myodocopida). J. micropalaeontol. 4(2), 159-1 64.
-and GOODAY, A.J.1977. Some observations on the vertical distributionand stomach content of Gigantocypris
muelleri Skogsberg, 1920 (Ostracoda,Myodocopina).I n Lofiler,H. and Danielopol, D. (eds.). Aspects of ecology
and zoogeography of Recent and fossil Ostracoda, 263-270. Junk, The Hague.
et al., 1984. RRS Discovery Cruise 148: 21 May-12 June 1984. Biological studies in the eastern North
Atlantic (48O-35ON). centred around the King's Trough Flank (42"WN; 2lo30'W). Inst. Ocecutogr. Sci., Cruise
Report, No. 163,31 pp.
TETART, J. 1978. Les garnitureschromosomiquesdes Ostracodes d'eau douce. Trav. Lab. Hydrobiol., 69-70,113-140.
This Page Intentionally Left Blank
Morphological and Ethological Adaptations of Ostracoda
to Microhabitats in Zostera Beds
Kunazawa University, Japan
Detailed sampling of Ostracoda in Zosteru beds revealed that two ostracod faunas are living in
two distinct microhabitats, namely the leaves of Zostera and the surface of the sand bottom. Common morphological characters are seen among the constituent species of each fauna. The phytal
species have round carapaces with a convex ventral area. Conversely, the sand bottom species have
elongated carapaces with a flat ventral plane. Through the observation of their behaviour, it has
been found that the differences in morphology are related to their mode of life, especially to their
copulatory behaviour, which has adapted differently to their respective microhabitats. The functions of other morphological characters of the ostracod soft parts have also been estimated on the
basis of their mode of life.
Adaptation is generally reflected in the morphology of animals as its functional aspects which
are attributed to evolution. An adequate functional appreciation of morphology should be based
on the study of the mode of life of the animals.
Little has been known about the mode of life of Ostracoda themselves. Previous works have revealed the relationships between habitats and faunal compositions of Ostracoda. This is exemplified
by the works of Williams (1969) and Whatley and Wall (1975) on phytal fauna in relation to microhabitats, its faunal composition being controlled by the species of algae. While carapace shapes
have been mainly described for ostracod taxonomy, some work has dealt with functional
aspects. Benson (1970, 1975) stressed the necessity of biomechanical interpretation of the carapace
shape. McGregor and Kesling (1969) discussed the functional morphology of ostracod carapaces
and soft parts in relation to their copulatory behaviour.
The aim of this work is to combine the characteristicfeatures of the microhabitats, mode of life
and morphology of Ostracoda living in the sea grass Zosteru beds of the shallow subtidal zone.
The Zosteru beds may offer several kinds of microhabitats for Ostracoda, though it can be regarded as a single habitat in a broad sense. We can consider the surface of the Zostera leaves,
the surface of the bottom sediments, the interstices of the sediment particles, etc., as possible microhabitats for Ostracoda. The first step in this work is to investigate what kind of species are
living in those microhabitats in the Zosteru beds. The second step is to comprehend the relationships between the morphologies and microhabitats through observation of the modes of life and
304 T. KAMIYA
to estimate the functional characters of ostracod morphology, especially that of carapace shape.
OF Zosteru BEDS
The eel grass, Zostera marina is one of the most popular and abundant sea grasses throughout
the world. The plant, consisting of several thin leaves, 50-100 cm in length and about 1 cm in width,
is joined to other plants by subsurface stems and grows in stock to form Zostera beds. A leaf has
a short life, falling off within about two months and being swept away from the beds. There is an
obvious seasonal vicissitude in the Zostera beds. The leaves grow thickly from spring to early
summer and decline from late summer to winter.
Samples were collected from the Zostera beds in Aburatsubo Cove which is located near the
southern tip of the Miura peninsula, Pacific coast of central Japan (Text-fig. 1). Aburatsubo Marine Biological Station of the University of Tokyo is located on shore. The water temperature in
the cove ranges from about 27°C (Aug.) to 8°C (Feb.).
Here, Zostera marina grows rankly in shallow water, 30 cm to 2 m deep during spring low tide,
along the shore near the mouth of the cove (Text-fig. 1). The substratum in the Zostera beds is composed of medium- to coarse-grained sand. The surface is covered with a soupy flocculent layer
less than about three millimeters thick.
Three micro-environments were postulated in these Zostera beds where the benthonic Ostracoda
might live. They are the surface of Zostera leaves, the surface of the sand bottom, i.e. in and on the
surface of the flocculent layer, and the subsurface interstices between the sand grains. The Zostera
leaves, which have smooth, flat surfaces, stand upright from the bottom because they hold air
I-Location of Aburatsubo Cove. Hatched area in the cove shows the distribution of Zosteru beds.
MMBS: Misaki Marine Biological Station of the University of Tokyo.