HELGOL,~NDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 34, 497-506 (1981)
O b s e r v a t i o n s o n t h e m o r p h o l o g y a n d v e g e t a t i v e c e l l
d i v i s i o n of t h e d i a t o m
D o n k i n i a r e c t a
E i l e e n J. Cox*
Biologische Anstalt Helgoland (Litoralstation);
D-2282 List/Sylt, Federal Republic of Germany
ABSTRACT: Wild and cultured material of
Donkinia recta
(Donk.) Grun. has been examined using light microscopy, and both the structure of the living cell and its vegetative cell division are described. Unlike most naviculoid diatoms,D. recta
has four chloroplasts per cell, each with four oblong pyrenoids, and division of the chloroplasts follows rather than precedes mitosis and cell division. These two unusual features are probably linked. The species is briefly discussed in relation to other naviculoid taxa in the light of these findings.I N T R O D U C T I O N
A l t h o u g h r a r e l y a b u n d a n t ,
Donkinia
r e c t a (Donk.) Grun. is a n e a s i l y r e c o g n i s e d diatom, o c c u r r i n g on s a n d y b e a c h e s a l o n g N o r t h S e a coasts. Its t a x o n o m i c t r e a t m e n t , h o w e v e r , has varied. S o m e authors e.g. v a n H e u r c k (1896), H e n d e y (1964) r e c o g n i s eDonkinia
Ralfs (Pritchard, 1861) as a v a l i d g e n u s , w h i l e others p l a c e its m e m b e r s p e c i e s inPleurosigrna
W.Sm. ( P e r a g a l l o & P e r a g a l l o , 1897-1908), or inGyrosigma
H a s s a l l (Cleve, 1894; V a n L a n d i n g h a m , 1969). S i m i l a r l y it has b e e n c o n s i d e r e d r e l a t e d toPleurosigma
a n dGyrosigma
on the o n e h a n d , a n d toTropidoneis
C l e v e a n dAmphiprora
C l e v e on the other. M e m b e r s ofDonkinia
h a v e a s i g m o i d r a p h e (cf.Pleurosigma
a n dGyrosigma)
a l t h o u g h t h e v a l v a r o u t l i n e is b a r e l y s i g m o i d , w h i l e t h e r a p h e is r a i s e d on a r i d g e (cf.Tropidoneis
a n dAmphiprora),
a n d in g i r d l e v i e w the cells are c e n t r a l l y constricted. H o w e v e r , t h e r e is v e r y little p u b l i s h e d i n f o r m a t i o n on t h e g e n u s , a n d l i v i n g m a t e r i a l has only b e e n d e s c r i b e d by Karsten (1899, 1928) a n d M e r e s c h k o w s k y (1902-1903). A l t h o u g h n e i t h e r i l l u s t r a t e d l i v i n g cells b o t h r e p o r t e d that t h e y h a v e four c h l o r o p l a s t s p e r cell, w i t h p y r e n o i d s a c c o r d i n g to M e r e s c h k o w s k y (1902-1903), w i t h o u t a c c o r d i n g to Karsten (1899, 1928). Both s u g g e s t e d thatDonkinia
is r e l a t e d toTropidoneis;
M e r e s c h k o w s k y (1902-1903) c o n s i d e r e d that it r e p r e s e n t s a t r a n s i t i o n fromNavicula
toTropidoneis,
Karsten (1928) p l a c e d it b e t w e e nTropidoneis
a n dAmphiprora
in his s y s t e m a t i c t r e a t m e n t of g e n e r a .T h e w i d e s p r e a d o c c u r r e n c e of
D. recta
on t h e shores of t h e W a d d e n S e a off t h e N o r t h Sea i s l a n d of Sylt, in a u t u m n 1980, a n d its s u b s e q u e n t g r o w t h in culture, f a c i l i t a t e d a d e t a i l e d i n v e s t i g a t i o n of its v e g e t a t i v e s t r u c t u r e a n d c e l l division, v a l v a r structure a n d*Present address: Botany School, South Parks Road, Oxford, OX1 3RA, U.K.
498 E i l e e n J. Cox
v a r i a t i o n over a p e r i o d of time. A d i f f e r e n c e i n the t i m i n g of chloroplast division in the cell cycle, c o m p a r e d to other n a v i c u l o i d d i a t o m s was discovered. T h e v e g e t a t i v e cell d i v i s i o n is therefore d e s c r i b e d a n d the p o s s i b l e s i g n i f i c a n c e of the v a r i a t i o n from the n o r m a l p a t t e r n discussed. T h e results of frustular s t u d i e s a n d l o n g term growth studies will be d e s c r i b e d a n d d i s c u s s e d e l s e w h e r e .
MATERIALS AND M E T H O D S
Living m a t e r i a l of D o n k i n i a recta w a s collected from a n u m b e r of sites a r o u n d the i s l a n d of Sylt, off the north w e s t coast of G e r m a n y , in the a u t u m n of 1980. Cells were g r o w n i n F2 m e d i u m (McLachlan, 1973) i n p l a s t i c petri dishes, at 15 °C i n a l i g h t - d a r k r e g i m e of 14 h light, 10 h dark, with light i n t e n s i t y b e t w e e n 1400 a n d 2000 lux.
E x a m i n a t i o n of l i v i n g cells was c o n d u c t e d u s i n g a Leitz O r t h o l u x microscope with s e a - w a t e r i m m e r s i o n objectives, a n d p h o t o g r a p h e d on Agfa Ortho film. Other material was fixed, e i t h e r i n d i l u t e g l u t a r a l d e h y d e or chromic acid, a n d cell contents e x a m i n e d , with or w i t h o u t the aid of cytological stains, e.g. a l i z a r i n g r e e n , D e l a f i e t d ' s h a e m a t o x y - Iin, n u c l e a r red a n d S u d a n IV.
RESULTS
V e g e t a t i v e c e l l m o r p h o l o g y
In v a l v e view, cells of D o n k i n i a recta are l i n e a r , with a c u t e to s u b - a c u t e , oppositely deflected apices. At each a p e x o n e v a l v e m a r g i n s u b t e n d s a l a r g e r a n g l e with the apical axis t h a n does the other, the r a p h e l y i n g closer to the former (Fig. 1: 1, 4). The raphe is s i g m o i d (Fig. 1: 1, 4) a n d r a i s e d u p o n a r i d g e for most of its l e n g t h , except n e a r the c e n t r a l n o d u l e . As a result, i n g i r d l e v i e w the more or less r e c t a n g u l a r o u t l i n e is c e n t r a l l y c o n s t r i c t e d (Fig. 2: 1). Valves are 7 0 - 1 5 0 ~m long, 10-20 ~m wide, a n d the girdle is u s u a l l y 20-30 ~m deep. T h e g i r d l e consists of two u n m a r k e d b a n d s (Fig. 3: 8-10; Fig. 4: 3), e a c h a b o u t 12 ~m wide, a n d the d e p t h of the frustule varies with the stage in the life cycle a n d the d e g r e e of o v e r l a p of the two b a n d s . I m m e d i a t e l y after cell division cells are a b o u t 20 ~m deep, just before the f o l l o w i n g cell d i v i s i o n they m a y b e almost 40 ~m deep, w h e n the b a n d o v e r l a p is m i n i m a l . It is p o s s i b l e to see that the v a l v e is finely striated a n d has a d e e p m a n t l e (Fig. 2: 1-3; Fig. 3).
O b s e r v a t i o n s o n t h e d i a t o m D o n k i n i a r e c t a 499
s t a i n i n g w i t h D e l a f i e l d ' s h a e m a t o x y l i n i n d i c a t e t h a t t h e s e a r e p y r e n o i d s . ( R a r e l y t h r e e or f i v e p y r e n o i d s p e r p l a s t i d w e r e o b s e r v e d . )
A s i n m o s t n a v i c u l o i d d i a t o m s t h e n u c l e u s l i e s i n t h e c y t o p l a s m i c c o n n e c t i o n a c r o s s t h e c e n t r e of t h e c e l t (Fig. 1 : 2). T w o l a r g e v a c u o l e s a r e p r e s e n t , o n e b e t w e e n t h e n u c l e u s a n d e a c h a p e x (Fig. 1 : 2,3). Oil d r o p l e t s m a y a l s o b e p r e s e n t , m o r e c o m m o n l y w h e n t h e c e l l s h a v e b e e n c u l t u r e d t h a n i n t h e w i l d .
C e l l d i v i s i o n
F o r t h e first f e w m o n t h s of g r o w t h i n c u l t u r e c e l l d i v i s i o n o c c u r r e d d u r i n g t h e l a t t e r p a r t of t h e d a r k p e r i o d , a l t h o u g h l a t e r m o r e c e l l s w e r e f o u n d d i v i d i n g d u r i n g t h e l i g h t p h a s e . B e f o r e t h e o n s e t of c e l l d i v i s i o n , t h e c e l l e x p a n d s to its m a x i m u m v o l u m e , t h e g i r d l e b a n d o v e r l a p b e i n g r e d u c e d to a l l o w t h i s , t h e c h l o r o p l a s t s b e c o m e l e s s l o b e d a n d m o r e r o u n d e d (Fig. 2: 2i a n d t h e p y r e n o i d s a r e l e s s o b v i o u s .
T h e p l a s t i d s of e a c h p a i r m o v e i n o p p o s i t e d i r e c t i o n s , o n e i n t o e a c h v a l v e (Fig. 2: 3-7), so t h a t t h e g i r d l e is v a c a t e d (Fig. 2: 0] a n d t w o e l l i p s o i d a l p l a s t i d s , o n e f r o m e a c h
l
j
1
...
2
3
4
Fig. 1. Light micrographs of valve vtews of D o n k i m a recta. All scale lines r e p r e s e n t 10 [ml. I: Sigmoid raphe and the lobes of the plastids under the valve, face. Tile valve striae al~, also visible. 2: Centrally located nucleus, with two large vacuoles, one b e t w e e n the nucleus and each apex. 3: Pyrenoids inclined into the cell lumen and to one apex along one side, the other on the opposite side of the valve. 4: Plastids during cell division in the valvar region, diagonally positioned across the
5 0 0 E i l e e n J. C o x
!
2
4
5
m7 ~!~ii'
~
1 0
11
~ i ~!~!ili~!!ii~iz~ ~
O b s e r v a t i o n s on the d i a t o m D o n k i n i a r e c t a 501 p a i r lie in e a c h v a l v a r r e g i o n (Fig. 2: 7). If s e e n in v a l v e v i e w t h e p l a s t i d s u n d e r e a c h v a l v e lie d i a g o n a l l y o p p o s i t e e a c h o t h e r w i t h t h e c e n t r a l c y t o p l a s m b e t w e e n (Fig. 1: 4). All four are c l e a r l y v i s i b l e in g i r d l e v i e w (Fig. 2: 6-8). T h e n u c l e u s is m o r e e a s i l y s e e n at the c e n t r e of the cell (Fig. 2: 7), often w i t h a c o n s p i c u o u s n u c l e o l u s (Fig. 2: 6, 8) a n d the d i c t y o s o m e s are m o r e o b v i o u s (Fig. 2: 8). This p h a s e u s u a l l y t a k e s a b o u t two hours.
In l i v i n g m a t e r i a l it is often difficult to see the c h r o m o s o m e s , b u t t h e m e t a p h a s e p l a t e lies at a slight a n g l e to the v a l v a r p l a n e (Fig. 2: 9 - 1 l ; Fig. 3: 1). D u r i n g a n a p h a s e c h r o m o s o m e s are m o r e e a s i l y s e e n (Fig. 2: 11, 12; Fig. 3: 2-4). I n v a g i n a t i o n of t h e p l a s m a l e m m a b e g i n s in a n a p h a s e (Fig. 2: 11, 12; Fig. 3: 2-5) a n d c y t o k i n e s i s f o l l o w s q u i t e q u i c k l y (Fig. 3: 6, 7), o f t e n c o m p l e t e d in 5 - 1 0 min. P l a s m a l e m m a i n v a g i n a t i o n is s e e n most c l e a r l y as it p r o c e e d s from p o l e to c e n t r e (Fig. 3: 2-6), b u t it is also m o v i n g in from side to side, i.e. c e n t r i p e t a l l y (Fig. 4: 4-6), not in a s c i s s o r - l i k e m a n n e r . As the central c y t o p l a s m b e c o m e s d u m b - b e l l s h a p e d at late a n a p h a s e a n d e a r l y t e l o p h a s e (Fig. 3: 5-7; Fig. 4: 5,6) the p l a s m a l e m m a b i s e c t s this a n d c y t o k i n e s i s is c o m p l e t e (Fig. 3: 7). W h i l e the d a u g h t e r n u c l e i pass t h r o u g h t e l o p h a s e to i n t e r p h a s e (Fig. 3: 8 - 1 I ) , the s h a p e of t h e c y t o p l a s m a r o u n d t h e m c h a n g e s , t h e e x t e n t of t h e c o n t a c t w i t h e a c h p l a s m a l e m m a i n c r e a s e s a n d cell w a l l d e p o s i t i o n b e g i n s (Fig. 3: 7-9). V e s i c l e s c a n b e s e e n in t h e r e g i o n of t h e p l a s m a l e m m a e as this p r o c e e d s (Fig. 3: 9 - 1 i ) .
As t e l o p h a s e is r e a c h e d a n d the silica w a l l is b e i n g l a i d d o w n , the c h l o r o p l a s t s m o v e so that t h e y lie o n e a l o n g e a c h side of the d a u g h t e r cells (Fig. 3: 8-10), t h e y e l o n g a t e a p i c a l l y (Fig. 3: 11), b e c o m e m o r e l o b e d (Fig. 3: 12; Fig. 4: 1) a n d d i v i d e t r a n s v e r s e l y at the m i d d l e (Fig. 4: 2,3) to restore the o r i g i n a l n u m b e r of four. As t h e p l a s t i d s r e g a i n t h e i r l o b e d outline, the p y r e n o i d s a g a i n b e c o m e c o n s p i c u o u s (Fig. 3: 12; Fig. 4: 1-3) s h o w i n g t h e s a m e o r i e n t a t i o n in both d a u g h t e r cells, a n d f i n a l l y t h e d a u g h t e r ceils s e p a r a t e . S i m u l t a n e o u s l y w i t h the r e s t o r a t i o n of n o r m a l c h l o r o p l a s t m o r p h o l o g y t h e n u c l e u s m o v e s from the h y p o v a l v a r r e g i o n to a m o r e c e n t r a l p o s i t i o n in t h e celt (Fig. 3: 11, 12;
Fig. 4: I, 2). F o r m a t i o n of the h y p o c i n g u l u m is the last s t a g e in t h e p r o c e s s a n d m a y o c c u r after cell s e p a r a t i o n . T h e c o m p l e t e s e q u e n c e , i.e. r e d i s t r i b u t i o n of chloroplasts, mitosis, cytokinesis, n e w v a l v e f o r m a t i o n a n d r e o r g a n i s a t i o n of the c e l l u l a r contents, t a k e s a b o u t 6'/2 h, of w h i c h o n l y 2 0 - 3 0 m i n is r e q u i r e d for mitosis a n d c y t o k i n e s i s .
D I S C U S S I O N
As m e n t i o n e d b e f o r e , D o n k i n i a r e c t a is u n u s u a l in h a v i n g four p l a s t i d s p e r cell, a n d in the t i m i n g of c h l o r o p l a s t d i v i s i o n in t h e cell cycle. A f e w o t h e r s p e c i e s are k n o w n w i t h four plastids p e r frustule, but t h e i r cell d i v i s i o n has not b e e n d e s c r i b e d .
Fig. 2. Light micrographs of girdle views of Donkinia recta. All scale lines represent 10 grn. Times of photographs given in brackets. 1: Vegetative cell showing the lobed plastids with the rectangular pyrenoids visible, inclined towards one apex. 2-7: Showing the migration of the chloroplasts and the pyrenoids, visible prior to mitosis. 2: Plastids rounded up. 3-7: Movement towards valvar regions, each plastid of a pair moving in opposite directions. (3:00 rain; 4 : t 2 rain; 5:25 rain; 6:38 min; 7:81 rain). 8, 9: Prophase - metaphase. 8: Cell in prophase, nucleolus, and dictyosomes near the nucleus in the epivalve, visible (00 min). 9: Metaphase plate at an angle to the valvar plane (01 min). 10-12: Metaphase - anaphase. I0', Metaphase with the chromosomes at the metaphase plate visible (00 min). 1I: Early anaphase - metaphase plate disappearing and the invagination of the plasmalemma beginning (05 min). 12: Anaphase - movement of the chromosomes to the poles (06
5 0 2 E i l e e n J. C o x
2
3
4
5
6
7
8
i
.9
1 0
f
11
O b s e r v a t i o n s o n t h e d i a t o m D o n k m i a rec&~ 5 0 3
4
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Fig, 4. L i g h t m i c r o g r a p h s of g i r d l e v i e w s . All s c a l e l i n e s r e p r e s e n t 10 !nn. T i m e s of p h o t o g r a p h s g i v e n in b r a c k e t s , l - 3 : Plastid d i v i s i o n , l: D e e p c e n t r a l i n v a g i n a t i o n of f h e p l a s t i d w s i b l e a n d fo/Jr p y r e n o i d s v i s i h l e in e d c h hdlf (164 fill[l), 2: D i v i s i o n of p l a s t i d s a l m o s t c o m p l e t e . . 2 : T w o d a u g h t e r c e l l s s h o r t l y b e f o r e t h e i r s e p a r a t i o n . 4 - 6 ; A n a p h a s e a n d c y t o k i n e s i s s h o w i n g v e s i c u l a r a c t i v i t y b e s i d e t h e p l a s m a l e m m a a n d t h e c e n t r i p e t a l m o d e of its i n v a g i n ~ t t i o n ( 4 : 0 0 m i n ; 5 : 0 1 rain; 6 : 0 3
rain)
Fig. 3 M i t o s i s arid cell d i v i s i o n . All s c a l e l i n e s r e p r e s e n t 10 tin1. T i m e s of p h o t o g r a p h s g i v e n in b r a c k e t s . 1: M e t a p h a s e , s b o w i n g t h e m e t a p h a s e p l a t e r e c l i n e d to t h e v a l v a r p l a n e (00 ruin). 2: E a r l y a n a p h a s e (05 rain). 3: A n a p h a s e - t h e c h r o m o s o m e s a r e v i s i b l e as t h e y m o v e to t h e poles. P l a s m a l e m m d i n v a g i n a t i o n b e g i n n i n g {07 rain). 4: A n a p h a s e - c h r o m o s o m e s a l m o s t at t h e p o l e s a n d i n v a g i n a t i o n p r o g r e s s i n g (08 rnin), 5: T e l o p h a s e i n v a g i n a t i o n a p p r o a c h i n g t h e c e n t r a l c y t o p l a s m {09 rain). 6: T e l o p h a s e - c y t o k i n e s i s a l m o s t c o m p l e t e (11 rain). 7: T e l o p h a s e -- c y t o k i n e s i s c o m p l e t e ; n o t e l h e v e s i c u l a r activity b e s i d e t h e p l a s m a l e m m a e (14 rain). 8: N u c l e i g o i n g into i n t e r p h a s e a n d walI d e p o s i t i o n b e g i n n i n g . T h e c o n t a c t b e t w e e n t h e c e n t r a l c y t o p l a s m a n d p l a s m a l e m m a ~n e,~ch cell is g r e a t e r anr_l c h l o r o p l a s t m o v e m e n t b e g i n n i n g a g a i n (25 m i n ) . f}: C o n t i n u a - tion of wall d e p o s i t i o n ~nd c h l o r o p l a s t r e a l i g n m e n t (28 rain). 10: O h t o i o p l a s t s m o r e cu l e s s in lice c e n t r e of e a c h g i r d l e , d a u g h t e r n u c l e i c o n d e n s i n g (36 rain), l l ; E l o n g a t i o n of c h l o r o p l a s t s o c c u r - ring, d a u g h t e r n u c l e i in i n t e r p h a s e a n d t h e c e n t r a l c y t o p l a s m n a r r o w e r (58 m i n ) . 12: C h l o r o p l a s t s
504 E i l e e n J. C o x
N o r m a l l y in n a v i c u l o i d d i a t o m s p l a s t i d d i v i s i o n occurs b e f o r e mitosis and w h e r e t h e r e is o n l y o n e p l a s t i d p e r cell c l e a r l y this is e s s e n t i a l , as c y t o k i n e s i s follows q u i c k l y after mitosis. U s u a l l y in a c e l l w i t h two p l a s t i d s o n e m o v e s to e a c h v a l v e a n d t h e y d i v i d e m o r e or less t r a n s v e r s e l y b e f o r e t h e start of mitosis. N u c l e a r a n d cell d i v i s i o n follow, a n d w h e n t h e s e are c o m p l e t e t h e p l a s t i d s in t h e d a u g h t e r cells m i g r a t e so that one lies a l o n g e a c h s i d e of t h e cell. O f t e n t h e t r a n s v e r s e d i v i s i o n of the p l a s t i d s is s o m e w h a t o b l i q u e (Geitler, 1979; Thaler, 1972; Cox, unpubl.), t h e b e h a v i o u r s h o w i n g a d e f i n i t e right- or l e f t - h a n d e d n e s s , as in o t h e r p h e n o m e n a a s s o c i a t e d w i t h r e p r o d u c t i o n in p e n n a t e d i a t o m s (Geitler, 1979; Rudzki, 1964). In D o n k i n i a t h e p o s i t i o n of t h e p l a s t i d s in e a c h v a l v a r r e g i o n p r i o r to c e l l d i v i s i o n is also d i a g o n a l , across t h e a p i c a l axis.
I
mifosisI ~ cyfokinesis
HTI-] p[asfid movemenf
FR:FI p[asfid division
l---I vegefafive
D
daughfer ce[kseparation
Fig. 5. Diagram to show the sequence of events in the cell cycle of a typical naviculoid diatom and of Donkinia recta. (Not to a time scale)
C h l o r o p l a s t d i v i s i o n p r i o r to mitosis a n d c y t o k i n e s i s is e s s e n t i a l in s p e c i e s w i t h only o n e p l a s t i d p e r v e g e t a t i v e c e l l if b o t h d a u g h t e r cells are to c o n t a i n chloroplasts, a l t h o u g h for cells w i t h two plastids, t h e i r d i v i s i o n could, t h e o r e t i c a l l y , o c c u r b e f o r e or after c y t o k i n e s i s . G e i t l e r (1977) r e p o r t e d o n e E u n o t i a s p e c i e s in w h i c h the c h l o r o p l a s t s d i v i d e after c e l l d i v i s i o n a l t h o u g h o t h e r F u n o t i a s p e c i e s s h o w p r e - m i t o t i c p l a s t i d division. W h e n , as in D o n k i n i a , four p l a s t i d s are p r e s e n t in t h e v e g e t a t i v e cell, p o s t - m i t o t i c d i v i s i o n m e a n s t h e r e are n e v e r m o r e t h a n four p l a s t i d s in a cell, w h i c h m u s t c e r t a i n l y s i m p l i f y r e d i s t r i b u t i o n of o r g a n e l l e s d u r i n g c e l l division. H o w such a c h a n g e in t i m i n g c a m e a b o u t is a m a t t e r of s p e c u l a t i o n .
O b s e r v a t i o n s o n the d i a t o m
Donkinia recta
505 plastids inDonkinia
r a r e l y m o v e c o m p l e t e l y u n d e r the v a l v e face as t h e y c a n i n other g e n e r a , e.g.Navicula, Gyrosigma,
b u t this is u n n e c e s s a r y i nDonkinia
b e c a u s e its d e e p v a l v e m a n t l e a l l o w s a c c o m o d a t i o n of the plastids, s u c h that the g i r d l e r e g i o n is n o t o c c u p i e d w h i l e cell d i v i s i o n is p r o c e e d i n g . Plastid m o v e m e n t is a l m o s t e n t i r e l y i n o n e p l a n e , a n d is s h o w n d i a g r a m m a t i c a l l y i n Fig. 6.1 2 3
4 5 6
Fig. 6. Diagram to show the movement of the plastids and their division. Stippling indicate the two plastids from one pair and their descendants
T h e n u m b e r a n d d i s t r i b u t i o n of chloroplasts i n
Donkinia
shows some s i m i l a r i t y toTropldoneis
(Cox, u n p u b l . ) , a l t h o u g h Schfitt (1896) r e p o r t e d that t h e latter h a d o n l y two plastids, a n d K a r s t e n (1899, 1928) r e p o r t e d two or four.Tropidoneis
also h a s its r a p h e raised u p o n a ridge, a l b e i t straight n o t sigmoid. Plastids w i t h h i g h l y d i s s e c t e d m a r g i n s h a v e b e e n o b s e r v e d inG. balticum
(Ehrenb.) C l e v e (Cox, u n p u b l . ) , a l t h o u g h the latter has o n l y two p e r cell, b u t the cells are s i g m o i d u n l i k eTropidoneis.
G e i t l e r (1937) d e s c r i b e d o b l i q u e l y o r i e n t a t e d p y r e n o i d s i n the plastids ofGyrosigma
species, b u t r e c t a n g u l a r p y r e n o i d s h a v e p r e v i o u s l y o n l y b e e n r e p o r t e d forhZIastogloia grevillii
W.Sm. (Stoermer et a1.,1964). T h e p y r e n o i d s i nDonkinia
are i n d i s t i n c t d u r i n g mitosis, p a r t i c u - larly b e t w e e n m e t a p h a s e a n d telophase, r e a p p e a r i n g c l e a r l y o n l y as the chloroplasts e l o n g a t e "and t h e n divide. T h e y are p r e s u m a b l y r e p l i c a t i n g d u r i n g this time, prior to plastid division, since before its c o m p l e t i o n four p y r e n o i d s are p r e s e n t i n e a c h half of the plastid. C o n f i r m a t i o n of this r e q u i r e s further i n v e s t i g a t i o n , p o s s i b l y i n v o l v i n g e l e c t r o n microscopy.5 0 6 E i l e e n J. C o x
A c k n o w l e d g e m e n t s . T h i s w o r k w a s c a r r i e d o u t w h i l e t h e a u t h o r w a s t h e h o l d e r of a F e l l o w s h i p f r o m t h e R o y a l Society, a s p a r t of t h e i r E u r o p e a n S c i e n c e E x c h a n g e P r o g r a m m e , a n d t h e i r s u p p o r t is g r a t e f u l l y a c k n o w l e d g e d . I s h o u l d a l s o l i k e to t h a n k t h e B i o l o g i s c h e A n s t a l t H e l g o l a n d for t h e i r h o s p i t a l i t y , Dr. G. D r e b e s for u s e f u l d i s c u s s i o n a n d i n t e r e s t in t h i s w o r k , a n d Ms. H. H a l l i g e r for t e c h n i c a l a s s i s t a n c e .
L I T E R A T U R E C I T E D
C l e v e , P. T., 1804. S y n o p s i s of t h e n a v i c u l o i d d i a t o m s . P a r t I. - K . s v e n s k a V e t e n s k - A k a d . H a n d l . 26,
1 - 1 9 4 .
Cox, E. J , 1981. T h e u s e of c h l o r o p l a s t s a n d o t h e r f e a t u r e s of t h e l i v i n g cell in t h e t a x o n o m y of n a v i c u l o i d d i a t o m s . In: P r o c e e d i n g s of t h e s i x t h i n t e r n a t i o n a l d i a t o m s y m p o s i u m . (In press.) G e i t l e r , L., 1937. D e r C h r o m a t o p h o r e n b a u d e r D i a t o m e e n Gyrosigma u n d Nitzscbia sigmoidea. -
Bot. Zhl. (Beih.) 57, 4 2 5 - 4 3 1 .
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M c L a c h l a n , J., 1973. G r o w t h m e d i a - m a r i n e . In: H a n d b o o k of p h y c o l o g i c a l m e t h o d s , c u l t u r e m e t h o d s a n d g r o w t h m e a s u r e m e n t s . Ed. b y J. R. Stein. U n i v . Press, C a m b r i d g e , 2 5 - 3 1 . M e r e s c h k o w s k y , C., 1 9 0 2 - 1 9 0 3 . L e s t y p e s d e I ' e n d o c h r o m e c h e z l e s D i a t o m S e s . - S c r i p t a bot. 2i,
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