En caninos

(With 2 figures in the text)

In A pril 1982, 77 h o u se m ice from th e O rk n ey Isla n d o f E d a y w ere released by R . J. Berry and his a sso c ia te s o n the Isle o f M a y , F irth o f F o rth . T h e Isle o f M a y h ad a sta n d a r d h o u se m o u se k a ry o ty p e (2n = 40), w h ile th o se fro m E d a y are h o m o z y g o u s for th ree cen tric fu sio n s (2 n = 34). W ith in 18 m o n th s o f in tro d u c tio n (S ep tem b er 1983), e a c h c en tric fu sio n h a d in creased in freq u en cy from an estim a te d sta rtin g v a lu e o f 8 % to a v a lu e c lo se to 50% , a n d th ey w ere seg reg a tin g in a cco rd a n ce w ith H a r d y -W e in b e r g e x p e c ta tio n s. In e sse n c e, th e tran sform ed p o p u la tio n w as b eh a v in g a s a p a n m ic tic un it. T h e freq u en cies o f in tro d u c ed c h r o m o s o m e s had a p p a ren tly sta b ilized by S ep tem b er 1986 w ith v a lu e s a r o u n d 65% fo r all th ree fu sio n s. T h e c y to g e n e tic d ata o b ta in e d in th e Isle o f M a y in tr o d u c tio n e x p e rim e n t accord w ell w ith d a ta for sin g le gen e loci (rep resen ted b y a llo z y m e d a ta ) a n d m o r p h o m e tr ic d a ta . M a le E d a y -M a y F I h yb rid s w ere fo u n d to h a v e a lo w freq u en cy o f n o n -d isju n ctio n (13% ). T h is stu d y is un usu al b eca u se th e su ccessfu l in tro d u c tio n o f m ice in to an e sta b lish e d p o p u la tio n , a n d th e in tro g ressio n and sta b iliza tio n o f th ree cen tric fu sio n s, c o u ld n o t h a v e b een p red icted fro m p r e v io u s stu d ies on the m o u se. Contents P age In tro d u ctio n ... 493 T h e Isle o f M a y ...49 4 M aterial a n d m e th o d s ...4 9 4 R esu lts ... ' . . 495 D i s c u s s i o n ... 499 R e f e r e n c e s ... 500 Introduction

Chromosome rearrangement may be important in spéciation. For example, related species often have different karyotypes. However, other well-defined species have the same karyotype (see

Mayr, 1963; White, 1978a,

b).

Central to the idea that chromosome rearrangements have a role in spéciation is that heterozygotes between chromosome races have reduced fertility, restricting gene flow, and leading to partial reproductive isolation. This may be followed by selection for premating isolation, and increased genetic distance through accumulated genetic change after spéciation (see Ayala, 1975).

‘ P resent address: S .E . T h a m es R e g io n a l G e n e tic s C en tre, 8th F lo o r , G u y ’s T o w er , G u y ’s H o sp ita l, St. T h o m a s ’ S treet, L o n d o n S E l 9 R T

493

4 9 4 P . N . S C R I V E N

There is uncertainty about the mechanism by which chromosome rearrangements become fixed within a population, and how several such rearrangements may be accumulated. Different factors which are thought to be important in fixation are: the selective advantage of the new homokaryotype; vagility; meiotic drive; inbreeding; genetic drift; and deme size (Bengtsson & Bodmer, 1976; Lande, 1979; Hedrick, 1981; Slatkin, 1981; Walsh, 1982).

Hybrid zones between chromosome races offer the opportunity to study evolutionary process in

nature. Many examples are documented including

Acomys

Gerbillus

(Wahrman & Gourevitz, 1973),

Sorex

Spalax

Thomomys

(Thaeler, 1974; Patton

et al.,

Uroderma

Areas of contact between races giving rise to Robertsonian polymorphism exist within the range

of the house mouse (Spirito

et ai,

et al.,

& Capanna, 1990; Nance

et al.,

and how important Robertsonian chromosomes are in maintaining such isolation. Also, studies on house mouse hybrid zones are quite likely to be confused by effects caused by the interaction of man and mouse. For example, mice can be passively transported over relatively large distances between farms in feed-stuffs.

This study describes the artificial creation of a house mouse Robertsonian polymorphism on a

small island. It is analogous to a hybrid area of contact between races of Robertsonian and all-

acrocentric mice, but essentially free of the disturbing influence of man. Perversely, because of the

success in introducing mice into an established population, it also shows that man can be effective in transporting mice over a large distance.

The Isle of May

In April 1982,77 house mice from the Orkney island of Eday were released on to the Isle of May,

Firth of Forth (60 ha; estimated population at that time was 1000 mice). Isle of May mice had 40 acrocentric chromosomes (2n = 40, N F = 40) and were allozymically monomorphic (and therefore probably highly inbred). Eday mice have three pairs of Robertsonian translocations (2n = 34, N F = 40) and 12 polymorphic allozyme loci have been described. Three centric fusions and 14

allozyme loci are now polymorphic and segregating in the Isle of May mouse population (Berry

et

al.,

Materials and methods

F ive hou se m ou se sam p les w ere iive-trap ped at random lo ca tio n s at different tim es after the in tro d u ctio n (see Berry

et ai,

S o m a tic k aryotyp e an a ly ses w ere co n d u cted o n G -b an d ed m etap hase sp reads o f peripheral b lo o d lym p h ocytes ob ta in ed by orbital sinu s pu ncture an d culture (B u ck lan d , E vans & Sum ner, 1971; T rim an, D a v isso n & R od erick , 1975). and o n bon e m arrow cells ob ta in ed by direct ch r o m o so m e preparation (F ord , 1966). T h e k aryotyp e w as determ ined from at least 3 high qu ality G -b a n d ed cells; an a d d ition al 3 cells were used for ch ro m o so m e co u n ts. M ale m eiotic p rep aration s (E vans, Breckon & F ord , 1964) were m ad e from original M ay, E day, and FI E d a y -M a y hybrids bred in the laboratory. In the m e io tic stu dy, n on -d isju n ction estim ates were m ade fo llo w in g the m eth od o f C attan ach & M oseley (1973). G -sta tistics were used in all cases to test for heterogeneity in frequency data (S ok al & R oh if, 1981).

C E N T R I C F U S I O N S I N T R O D U C E D I N T O I S L A N D M I C E

Results

4 9 5

In total, 302 mice were scored in five samples caught each September from 1983 to 1987 inclusive. There were no significant differences in chromosome frequencies between males and females. Chromosome frequencies for pooled sexes of all post-introduction samples are given in Table I. There were no significant deviations from Hardy-Weinberg expected frequencies at the 5% probability level.

Table I

F requ en cies o f c h ro m o so m es a n d a llo z y m e a lle le s' in tro d u c e d on to the Isle o f M a y . N is th e n u m ber o f m ice. E E , E M a n d M M a re E d a y , h y b r id a n d M a y ty p e

F u s io n /a lle le N EE EM M M p E

95% c o n fid en ce lim its for p E (b in o m ia l d ist.) S ep t. 1983 R b ( 3 .l4 ) 70 17 31 22 0 4 8 0 3 9 - 0 57 R b (4 .1 0 ) 70 16 33 21 0 4 6 0 3 8 - 0 55 R b (9 .1 2 ) 70 19 33 18 0 51 0 4 2 - 0 59 Ej-3'’ 65 17 29 19 0 4 8 0 4 0 - 0 57 Es-9* 68 16 42 10 0 54 0 4 4 - 0 65 E s - l l * 70 17 39 14 0 52 0 4 4 - 0 61 P g m -y -^ 70 16 36 18 0 4 9 0 4 0 - 0 57 P o o le d 483 118 243 122 0 50 S ep t. 1984 R b (3 .1 4 ) 51 13 29 9 0 54 0 4 5 - 0 65 R b (4 .1 0 ) 51 10 30 11 0 4 9 0 3 9 - 0 59 R b (9 .1 2 ) 51 26 21 4 0 72 0 6 2 - 0 81 P o o le d 153 49 80 24 0 5 8 S ep t. 1985 R b (3 14) 43 15 22 6 0 6 1 0 4 9 - 0 71 R b ( 4 1 0 ) 43 16 22 5 0 6 3 0 5 2 - 0 73 R b (9 .1 2 ) 43 25 14 4 0 74 0 6 4 - 0 83 P o o le d 129 56 58 15 0 6 6 S ep t. 1986 R b (3 .1 4 ) 59 22 30 7 0 6 3 0 5 4 - 0 72 R b (4 .1 0 ) 59 27 28 4 0 70 0 5 9 - 0 78 R b (9 .1 2 ) 59 22 32 5 0 6 4 0 5 4 - 0 73 £ j - 3 ” 59 31 17 11 0 6 7 0 5 7 - 0 74 Es-9* 59 14 33 12 0 52 0 4 2 - 0 65 E s - l l * 59 25 27 7 0 6 5 0 5 5 - 0 78 P g m -3 * '’ 59 24 27 8 0 6 4 0 -5 3 -0 -7 7 P o o le d 413 165 194 54 0 6 3 S ep t. 1987 R b (3 .1 4 ) 79 30 32 17 0 5 8 0 -4 9 -0 -6 5 R b (4 .1 0 ) 79 39 35 5 0 72 0 -6 3 -0 -7 9 R b (9 .1 2 ) 79 35 36 8 0 6 7 0 -5 8 -0 -7 4 P o o le d 237 104 103 30 0 6 6

6