A fragile site in the X chromosome of the reindeer (Rangifer tarandus L)

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A fragile site in the X chromosome of the reindeer

(Rangifer tarandus L)

U Gripenberg, S Huuhtanen, M Wessman, M Nieminen

To cite this version:

A

_fragile

site in

the X

chromosome of the

reindeer

_(Rangifer

tarandus

_L)

U

_Gripenberg

S

Huuhtanen

M

Wessman

M

Nieminen

1

Ilniversit

y

of Helsinki, Department of Genetics,

Arkadiankatu

_7,

SF-00100 Helsinki

_10;

2

Finnish

Game and Fisheries Research

_Institute,

Reindeer Research, SF-96100 Rovaniemi, Finland

(Proceedings

of the 9th

_{European Colloquium}

on

_Cytogenetics

of Domestic

_Animals;

Toulouse-Auzeville,

10-13

_July

₁₉₉₀₎

fraX

_{/ fragile}

site

_/

reindeer

_/

BrdU induction

INTRODUCTION

During

an

_{investigation}

on the heterochromatin in the reindeer

(Rangifer tarandus)

and some related

_species

(Alces

_alces,

Odocoileus

_virginianus

and Dama

_dama)

(Bianchi

et

_al,

_1990),

_{bromodeoxyuridine}

_(BrdU)

was added to the

_lymphocyte

cultures for the last 5 or 7 h of

_growth.

In chromosome

preparations

from BrdU-treated

_cultures,

a

_fragile

site

_(fra)

was observed in the X chromosome of the

reindeer. The X chromosome of the reindeer is

_{exceptionally}

_large

and _very different from the X chromosome of other mammals. The fra site was located at the distal end of one of the arms of the

nearly

metacentric X chromosome. The fraX seemed to

occur in a

_high

_proportion

of the BrdU-treated cells. fra sites were not observed in any of the other

species

investigated.

The accidental

finding

of fraX in the reindeer

initiated a reexamination of slides from the reindeer cell cultures.

The chromosomes of 5 reindeer were

investigated.

Three

subjects represented

the semi-domesticated animal

_(Rangifer

tarandus

_tarandus),

two were wild forest

reindeer

_(Rangifer

tarandus

_fennicus).

As has

_previously

been shown

_(Gripenberg,

1984),

no

_cytogenetic

differences _appear to exist between the two

_subspecies.

BrdU-treated cultures and

_parallel

cultures without BrdU were screened.

_Furthermore,

a sixth reindeer

_(Rangifer

tarandus

_fennicus)

was

investigated.

In this case

only

chromosome

_preparations

from cultures without BrdU were screened.

Lymphocytes

were cultured

according

to standard methods. Pokeweed

_mitogen

(PWM)

was the

preferred mitogen.

BrdU

(20 pg/ml)

was added to the cultures for the last 5 or 7 h of

_growth.

The chromosomes were G- and C-banded or stained

with conventional Giemsa. Whenever

_possible,

100

_metaphases

were screened for

the presence of fraX.

Figure

1 shows the G- and

_subsequently

C-banded chromosomes of the reindeer

the total

_complement,

while the mammalian X

_{usually comprises}

5%

of the genome

(Fraccaro

et

_al,

_1968).

Measurements of the

_length

of the arms of X showed

_Xq

to be

58%

of the total

length

of

_{X; large}

blocks of heterochromatin -

25%

of the chromatin of the X chromosome - were located in

_Xq

(fig 1).

The

_length

of

_Xp

was

42%

of

_X;

the

fra site was at the distal end of

_Xp

_{(fig 2).}

It is worth

_noting

that without the

acquisition

of the

_{heterochromatin,}

the relation between the arm

lengths

would be

the

_opposite:

the fraX arm would be the

_long

_arm,

_Xq.

Due to extensive structural

_{rearrangements}

in the X chromosome of the

reindeer,

bands or

_segments

_homologous

with the human X can

_hardly

be identified on the

basis of

_cytological

characteristics.

Table I shows the

_frequency

of fraX in cells from 6 animals treated with BrdU for

_0,

5 or 7 h. In cultures without _any added

_BrdU,

cells with fraX were

_only

occasionally

observed. Addition of BrdU for the last 5 h of

_growth

_apparently

enabled

_{incorporation}

into a smaller

_proportion

of the

_cells;

fraX was observed at a

frequency

less than

50%.

Cells

_{incorporating}

BrdU for 7 h showed a fraX

_frequency

in the _range of

53-86%

_{(mean 66%).}

In

_females,

the fra site is

_expressed

in both X chromosomes in about half of the cells. fraX is often seen in both sister chromatids.

fra sites have so far been described in

_only

a few animals

(Sutherland

and

Ledbetter,

1988).

In human

cytogenetics,

more than 100 fra sites are

_presently

known and most of these sites are

_regarded

as harmless chromosome variants. A

notable

_{exception is,}

_however,

the human fraX located at the distal end of the

_long

arm

_(Xq27.3)

and associated with transmissible mental retardation

_(Kdhk6nen,

1989).

The

_{cytological similarity}

between the human fraX and the reindeer fraX is obvious. In

_reindeer,

the

_original

chromosome arm with the distal fra site could be

site associated with mental retardation. The reindeer

fraX, however,

is a common

BrdU-induced fra site. As far as it is

_known,

the reindeer with fraX are normal in

their appearance as well as in their behavior.

The

_{biological significance}

of the fra sites is not known. It

has, however,

been

pointed

out that fra sites are

_cytogenetic

manifestations of normal structural

elements. Their

_significance

for chromosome evolution has been discussed

_(Mir6

et

al,

1987).

fra sites can be

regarded

as

_{recombinogenic}

hot

_spots

_causing

inter- as well as intrachromosomal

changes.

Whether the fra site of the reindeer X has contributed in some _way to the

structural

_changes

in the chromosome arm in

_question

is not known at

_present.

However,

the view that fraX in the reindeer is located at a site

corresponding

to

the site of the human fraX seems

_{possible. Homologies}

between human and reindeer

X chromosomes remain to be further

_{investigated.}

ACKNOWLEDGMENTS

Drs MS and NO Bianchi have worked

_together

with us on the same reindeer material

and have also observed the fraX in the reindeer. Dr NO Bianchi has read the

_manuscript

and has made valuable comments for which we are most

_grateful.

This work has been

supported by

the

_Academy

of Finland

_{(SA O1/101),}

the Societas Scientiarum Fennica and the Finnish Cultural Foundation. We _express our sincere thanks to Ms

_Tarja

Vilimiiki and

Ms Barbara von

_{Schoultz, MSc,}

for skillful technical assistance. Helsinki Zoo has

_provided

us with blood

samples

from the wild forest

_reindeer,

for which we are most

_grateful

to Mr H Jalanka DVM.

REFERENCES

Bianchi

MS,

Bianchi

_NO,

_Gripenberg

_U,

Wessman

_M,

Huuhtanen S

₍₁₉₉₀₎

Characteriza-tion of the heterochromatin in moose

_{(Alces alces)}

chromosomes. Genetica

80,

1-7

Fraccaro

M,

Gustavsson

_I,

Hult6n

M,

Lindsten

J,

Tiepolo

L

₍₁₉₆₈₎

_Chronology

of DNA

replication

in the sex chromosomes of the reindeer

(Rangifer

tarandus

L).

_Cytogenetics

_7,

Gripenberg

U

₍₁₉₈₄₎

Characterization of the

_karyotype

of the reindeer

_{(Rangifer tarandus);}

the distribution of the heterochromatin in the reindeer and the Scandinavian moose

(Alces

alces).

In: Proc 6th Eur

_{Colloq Cytogenet}

Domest Anim Zurich.

_(Stranzinger

_G,

_ed)

68-79

Kahkonen M

₍₁₉₈₉₎

_{Population cytogenetics}

of folate-sensitive

_fragile

sites with

_special

reference to mental retardation.

Thesis, University

of

_{Oulu, Printing Center,}

Oulu Mir6

_R,

Clemente

IC,

Foster

_{C, Egozcue}

J

₍₁₉₈₇₎

_Fragile

sites, chromosome evolution and human

neoplasia.

Hum Genet 75, 345-349

Sutherland

_GR,

Ledbetter D

₍₁₉₈₈₎

_Report

of the Committee on

_Cytogenetic

Markers.