RESULTADOS Y DISCUSION

(With 6 figures in the text)

In A pril 1982, h o u se m ice from the Island o f E d ay (O rk n ey ) w ere in tro d u ced o n to th e Isle o f M ay (F irth o f F o rth . S co tla n d ) (B erry. T rig g s. K in g . N a sh & N o b le . 1991). T h is p ro v id ed an op p o rtu n ity to stu d y h yb rid ization b etw een m o r p h o lo g ic a lly d istin ct p o p u la tio n s o f the h o u se m o u se. Sign ificant ch a n g es du e to in h erited fa cto rs w ere fo u n d after o n ly 18 m o n th s. G o o d agreem ent w as fo u n d betw een sin gle gen e lo c i (rep resen ted by a llo z y m e d a ta ) and m o r p h o lo g y o f the m an dible). C a n o n ica l d iscrim in a n t a n a ly sis sep a ra ted the sa m p le s q u ite clea rly (F = 139 9, f < 0 01) w ith h ig h ly sign ifican t M a h a la n o b is gen era lized d ista n ce s ( f < 0 0 1 ). P rin cip al co m p o n en t a n a ly sis clearly resolved size (F = 53 0, P < 0 0 1 ) an d sh a p e (F = 68-3, f < 0 01) differences b etw een the sam p les and sh o w ed p o st-in tr o d u c tio n m ice w ere sig n ifica n tly larger than E day and p re-in tro d u ctio n M ay m ice, and in term ed ia te in m a n d ib le sh a p e. T h e size d ifference is a m u ltivariate in d ica tio n o f h eterosis. T h e sh a p e d ifferen ces d escrib ed th e c o r o n o id p ro cess and in cisor region. A v era g e p o st-in tr o d u c tio n m ice w ere 6 -8 % larger th an th e a v era g e E d a y -M a y m id -parent value. M u ltivariate p o st-in tr o d u c tio n m a n d ib le v a ria n ces w ere sig n ifica n tly greater than p re-in tro d u ctio n variances (Sni F = 2 -I9 , P < 0 01). O f the to ta l p o st-in tr o d u c tio n m a n d ib le va riation . 54% w a s due to g en etic d ifferen ces. A llo z y m e and m a n d ib le sh ap e d a ta are in g o o d agreem ent. A hyb rid index using six b lo o d a llo z y m e s (d ev ised to in v estig a te in tro g ressio n at the level o f in d ivid u al m ice). N e i's gen etic d ista n ce c a lc u la te d w ith 19 a llo z y m e lo ci, a n d m a n d ib le sh ap e, placed p o st-in tr o d u c tio n M a y m ice p recisely m id -w a y b e tw een E d a y and p re-in tro d u ctio n M a y . P o st in tro d u c tio n m ice w ere m ore v a ria b le e le ctro p h o r etica lly a n d m o r p h o lo g ic a lly than p re-in tro d u ctio n M a y m ice. T h is stu d y sh o w s u n eq u iv o c a lly th a t h y b rid iz a tio n ca n be a con sid era b le fa cto r in the d ifferen tiation o f natural p o p u la tio n s.

Contents

Page In tro d u ctio n ... 573 M aterial and m e th o d s ...574 R esu lts ... 575 C o m p a riso n w ith electro p h o retic data ... 579 D i s c u s s i o n ... 582 R e f e r e n c e s ... 583

Introduction

In April 1982, 77 house mice from the Island of Eday in the Orkney archipelago were introduced

on to the Isle of May in the Firth of Forth, Scotland (Berry, Triggs, King. Nash & Noble, 1991).

' Present address: S.E. T h a m es R egion al G en etics C en tre. G u y ’s F lo sp ita l. G u y ’s T o w er 8th F lo o r , St T h o m a s S treet, L o n d o n SE I 9 R T . U K

- U n i\e r sité C a th o liq u e de L o u v a in . U n ité d e B io lo g ie A n im a le. 5 Place C roix du S u d . 1348 L o u v a in -la -N e u v e , B elg iu m 573

574 P. N . S C R I V E N A N D V . B A U C H A U

Introduced allozyme alleles and Robertsonian translocations (Berry

et al.,

spread very quickly. Within 18 months the frequencies of introduced alleles and fusions had

approached 50% of their Eday value, and every mouse scored had at least one introduced allele or

fusion.

Allozymically house mice are among the more variable mammalian species; Berry (1981) quotes an overall average heterozygosity per locus (H) of 7%. Surprisingly, samples of May mice have had an H value of zero (Berry, 1981). In one survey involving over 70 allozyme loci, only five

heterozygotes at two loci (xylose dehydrogenase-1 and transferrin) were found (Berry

et al.,

On the other hand, Eday mice have considerable allozymic variation with H = 6-4% for 76 loci

screened in 96 mice (Berry, unpubl.). By September 1983 (within 18 months of introduction), the

May population (which consisted entirely of hybrids) had an H value of 7-3%. Fourteen loci are

now known to be polymorphic on the Isle of May compared to 12 on Eday and none on the Isle of

May pre-introduction.

The isolated nature of the Isle of May makes it very suitable for study. It is uninhabited except for summer bird-watchers and lies 10 km off the east (Fife) coast of Scotland. There is a very low likelihood of immigration of animals. Apart from mutation, post-introduction allozymic variation can only be a consequence of introducing mice.

The Isle of May experiment has provided an excellent opportunity to investigate the effect of hybridization on morphology in a natural population of house mice.

Material and methods

F ive hou se m ou se sam ples were used in this stu dy. T w o sam p les were live-trap ped using L o n g w o rth small m am m al traps on the Isle o f M ay before the in trod u ction . T hey were su b seq u en tly p o o led o n findin g no statistically significant difference betw een them (49 an im als). T he other sam p les were: 1 rick-caugh t sam ple from E day (from the sam e farm as the in trod u ced m ice) (67 anim als), and 2 sam p les live-trap ped o n the Isle o f M a y 18 m on th s (40 anim als) and 54 m o n th s (39 an im als) after the in trod u ction .

T he m andible w as chosen for study b ecau se o f its relatively fiat shape and the ease with w h ich repeatable m easurem ents can be m ade. T he m and ib le p rovid es in form ation on both size and shape. M an d ib le sh ape was used very su ccessfu lly by T esting (1972) in id en tifyin g different inbred strains o f m ice, and by D a v is (1983) w h o applied T estin g’s m eth ods to distin gu ish betw een different p op u la tio n s o f wild British h o u se m ice. On these (and oth er ground s) m and ib le size and shape h ave been con clu d ed to be m od erately to high ly inherited characters in the laboratory m ouse (T h orp e, 1981; T h orp e & Leam y. 1983).

M o u se h eads w ere boiled for 40 min in tap w ater before in cu b ation in a so lu tio n o f com m ercially availab le laundry pow der (‘B io-tex’) at 37 C for several days (after D a v is, 1983). F lesh rem oved, the b o n es w ere then w ashed in tap w ater and dried overn igh t at the sam e tem perature.

In order to m inim ize overall size differences, o n ly anim als assign ed to dental w ear stages 3 and 4 (L idicker, 1966) were used in the study. T his represents m ice o f ap p roxim ately 6 0 -1 8 0 d ays old. T or each sam p le, no difference in eith er size or shape w as foun d betw een m ice o f age class 3 and age class 4. H o w ev er, it sh o u ld be noted that the p o st-in tro d u ctio n m ice used in this an alysis m ay be slightly older than their m olar w ear w ou ld su ggest. U n lik e pre-in trod uction M ay an d E day m ice w h ich were killed w ithin 10 days o f capture, p o s t­ in trod u ction M ay m ice were kept for 3 m o n th s w h ilst ch ro m o so m e stu dies w ere carried ou t. W e d o n ot believe that this has had any co m p licatin g effect for the fo llo w in g reason. G reen & T ekete (1933) investigated differential grow th in the m ouse. T hey m easured m any different bon es inclu d in g the scapu la, tibia, radius, ulna, m etacarpals and femur. T hey co n clu d ed that all these b on es had attained their ap p roxim ate m axim um length by 91 d ays, with the m o st rapid rate o f grow th com p leted by 31 days o f age. M ore recently, a sim ilar result w as sh o w n for the m andible w ith little grow th after 91 d ays (L ovell. T o tm a n & J o h n so n , 1984).

C H A N G E S I N M O U S E M A N D I B L E M O R P H O L O G Y 575