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Chapter 24. Cytogenetic studies on marine myodocopid Ostracoda: The karyotypes of Gigantocypris dracontovalis Cannon, 1940 and Macrocypridina castanea (Brady, 1897)

Chapter 24. Cytogenetic studies on marine myodocopid Ostracoda: The karyotypes of Gigantocypris dracontovalis Cannon, 1940 and Macrocypridina castanea (Brady, 1897)

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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

swimming juveniles.

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.



RESULTS

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

and teleophase.

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.



296



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).



DISCUSSION

Present Material

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

OF THE

3 SPECIES

OF CYPRIDINACEA

(MYODOCOPINA)

Species

Chromosome Type

Gigantocyprismuelleri

Gigantocypris dracontovalis

Macrocypridina castanea



Diploid Chromosome Numbers

Metacentric

Submetacentric

18

12

6

8

8



Acrocentric



-



2



CHROMOSOMES OF



Size Range

(rum)

19-24

16-22

9-19



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.

TABLE

2-DISnUeVnON OP TYPES, DIPLOID

NUMBER

AND SIZERANGE

OF cHROMosoMEs WITHIN THE P O m P I N A

[BASED

ON DATA TAKEN FROM TETART

(1978)].

Superfamily

Cytheracea



Tatal(2n)

14



Diploid Chromosome Numbers

Metacentric

Submetacentric

2

-



Size Range

Acrocentric

12



(rum)



3



[Limmcythere (L.) inopinata]



bisexual

Darwinulacea



22



-



-



22



0.5-1



1-14



-



7-30



0.5-6



(Darwinulastevensoni)



parthenogenetic

Cypridacea

15-35

(various species)

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

future.

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.



ACKNOWLEDGEMENTS

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.



REFERENCES

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.

ROE, H.S.J.

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.



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Morphological and Ethological Adaptations of Ostracoda

to Microhabitats in Zostera Beds

TAKAHIRO

KAMIYA

Kunazawa University, Japan



ABSTRACT

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.



INTRODUCTION

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

303



304 T. KAMIYA



to estimate the functional characters of ostracod morphology, especially that of carapace shape.



THEENVIRONMENT

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



TEXT-FIG.

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.



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Chapter 24. Cytogenetic studies on marine myodocopid Ostracoda: The karyotypes of Gigantocypris dracontovalis Cannon, 1940 and Macrocypridina castanea (Brady, 1897)

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