Paracentric inversion involving NOR of chromosome 8 in a boar : studies of synaptonemal complexes under a light microscope

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Paracentric inversion involving NOR of chromosome 8 in

a boar : studies of synaptonemal complexes under a light

microscope

M Świtoński

To cite this version:

Original

article

Paracentric

inversion

_involving

NOR

of chromosome

8

in

a

boar :

studies of

_{synaptonemal complexes}

under

a

light microscope

M

&jadnr;wito&jadnr;ski

Academy of Agriculture, Department of

Genetics and Animal

Breeding, Wolynska,

33,

Pl-60-6 3

7 Poznan, Poland

(Received

13 June _1990;

_accepted

28 _January

₁₉₉₁₎

Summary - In the

_karyotype

of a boar used in artificial insemination a

_paracentric

inversion :

_{inv(8)(pll, pl2), involving approximately}

8% of the chromosome

_length,

was

identified. The inversion

_split

the nucleolar

_organizer

_region

_(NOR)

into 2 _segments. .-!n

_analysis

of

_{synaptonemal complexes}

carried out under a

_light

_microscope revealed that in a

_majority

of

_primary

_{spermatocytes}

(87.2%)

_{heterosynaptic}

_{"straight pairing"}

in

_pair

No 8 occurred. In the

_remaining

cells

_{delayed pairing}

was found in this

_pair.

The

_loop

formation was not identified in any of the studied _{spermatocytes.} In

_addition,

no

_nonspecific

associations between sex bivalent and autosomal bivalents were found.

Consequences

of the observed

_pairing

behavior on

fertility

are discussed.

swine

/ chromosomal aberration

/ inversion

/ synaptonemal

complex

Résumé - Inversion _{paracentrique impliquant l’organisateur} nucléolaire du

chromo-some 8 chez un _verrat; étude des _{complexes synaptonémiques} au _microscope optique.

Dans le _caryotype d’un verrat utilisé en insémination

_artificielle

une inversion paracen-trique,

inv(8)(pll, p12),

impliquant approximativement

8% de la

longueur

du chromosome, a été

identifiée.

L’inversion _partage la

_région

de

_{l’organisateur}

nucléolaire

_(NOR)

en 2

segments.

L’analyse

des

_{complexes synaptonémiques}

réalisée au

_{microscope optique}

révèle que chez la

majorité

des spermatocytes

primaires

(87,2%)

l’hétérosynapsis

de la

_paire

n°8 8

intervient

rapidement.

Dans le reste des

cellules, l’appariement

pour cette

paire

est retardé.

Aucune

_formation

de boucle d’inversion n’a été observée chez les _{spermatocytes} étudiés.

En _outre, aucune association non

_spécifique

n’a été trouvée entre des bivalents sexuels et des bivalents

_{autosomiques.}

Les

_{conséquences}

_pour la

_fertilité

du mode

_{d’appariement}

observé dans cette étude sont discutées.

INTRODUCTION

Extensive

_cytogenetic

studies carried out in

_pigs

have revealed that the most

frequently

observed chromosomal aberrations are

_reciprocal

translocation

_(Popescu

et

_{al, 1984;}

_Long,

_1988).

_Up

till now there has been no

_report

_concerning

any case

of inversion in this

_species.

The existence of an inversion in a

_karyotype

can be a cause of

_pairing

disturbances at the

_pachytene

_substage

of meiotic

_prophase

I. The

_accepted

view is that

during

the

_pairing

_stage

_loop

formation is

_always

_present.

_Loop

formation allows

establishing homologous

pairing

between the inverted

_segment

of one chromosome

and the

_unchanged

_segment

of the

_homologue.

This can lead to

_crossing

over within

the

_{loop and,}

_{in consequence,} to the

_production

of unbalanced

_gametes.

On the other

_hand,

lack of

_loop

formation results in

_supression

of

_crossing

over within the

inverted

_segment

due to lack of

_homology

and the

_production

of

_only

balanced

gametes

(Kaelbling

and

_{Fechheirner, 1985; Hale,}

_1986).

This means that

depending

on

_{pairing behavior,}

variable effects on

_fertility

in carriers of inversions can be

found.

This _{paper presents} the first case of a

_paracentric

inversion in

_swine,

with

_special

attention to the

_analysis

of

_{synaptonemal complexes}

under a

_light

_microscope.

MATERIAL AND METHODS

Cytogenetic

studies were carried out on a boar of the Polish Landrace breed. The animal was selected from _among other boars used in artificial insemination.

Mitotic chromosomes were studied

_by

_application

of conventional

_staining

and

banding techniques :

GTG

_{(Seabright, 1971)}

and

_Ag-NOR

_(Bloom

and

Goodpas-ture,

1976).

For

_{karyotype analysis}

the international standard was used

(Committee

for the Standardized

_Karyotype

of the Domestic

_{Pig, 1988).}

Synaptonemal

complexes

were obtained

_by

the use of a

_{spreading technique}

(Counce

and

_Meyer,

₁₉₇₃₎

for

_light

_microscope.

The

_suspension

of meiotic cells

was

_spread

on

_drops

of 0.2 N sucrose. The air-dried

_drops

were fixed with

4%

paraformaldehyde

(pH 8.5),

rinsed in

0.4%

Photo-flo

_(Kodak,

_pH

_8.5)

and stained with

50%

AgN0

3

in a moist chamber

(60°C)

for 2 h.

RESULTS

Mitotic chromosomes

Chromosome

analysis

carried out

_by

_application

of conventional and

_banding

stainings

showed in chromosome No 8 an additional chromatin

_fragment

within

the nucleolar

_{organizer region}

_(NOR).

Silver

_staining

revealed in this chromosome

a double silver

deposit.

Detailed

_analysis

of the

_karyotype

indicated that in one

chromosome from the No 8

_pair

a

_paracentric

inversion had occurred. The

_breakage

points

were localized within the nucleolar

_{organizer region}

and

_just

above the

_region.

Synaptonemal complexes

Altogether

47

_primary

_{spermatocytes} were

_analyzed

under a

_light

_microscope.

All cells were

_complete

and each consisted of 18 autosomal bivalents and an XY bivalent

(fig 2).

The sex bivalent showed variable behavior and 4

_{morphological}

_types

were

distinguished

(table I).

Types 1,

2 and 3 were classified

_according

to the behavior

of the free ends of both sex chromosomes in the

_{following way :}

_1),

no contact

between

_them;

_2),

the free ends are associated

_end-to-end;

and

_3),

the free ends

are

_synapsed.

_Type

4 occurred when the axes of the sex bivalent were

_tangled.

An

_analysis

of autosomal bivalents showed that in the

_majority

_{of spermatocytes}

(40

cells)

all were

_{fully paired.}

In the case of 7

_{spermatocytes}

one autosomal bivalent

was

_{partly unpaired}

(figs

_3,

4).

Measurements of the

_length

of all the autosomal

bivalents within such cells showed that the bivalent with an

_unpaired

_segment

had the fourth or fifth

_{longest length.}

This

_{position corresponds}

to the

_length

of chromosome 8 obtained in mitotic chromosomes

_(Fries,

_1982;

Vth Committee for

the Standardized

Iiaryotype

of the Domestic

Pig,

1988).

Delayed pairing

in

_pair

No 8 occurred in spermatocytes which had a sex bivalent of

_{morphological}

_type

1 or 2

_(table

_I).

No

_nonspecific

associations between the sex bivalent and autosomal bivalents were found.

DISCUSSION

The

_paracentric

inversion described in this _{paper is} most

_likely

the first case

of such

_{rearrangement}

found in

_pigs.

The inverted chromosomal

_fragment

was

rather small and involved M

8%

of the

_length

of chromosome 8 alone. It is an open

question

whether the

length

of the inverted

fragment may

be

responsible

for

this

_pairing

behavior. In the case of inversion carriers this behavior is subdivided

into 3

_{stages :}

_{asynapsis, loop}

formation and

_{heterosynaptic &dquo;straight pairing&dquo;.}

The classical sequence of events

_during

late

_zygotene

and

_pachytene

starts with

asynapsis

and is followed

_{by loop}

formation

and,

in late

_pachytene,

the size of the

loop

decreases and

_{finally heterosynaptic}

_{&dquo;straight pairing&dquo;}

is established. In

_fact,

such a

regular sequence

of events was found to be not _very common. In some cases

such a _sequence was observed

(Davisson

et

al,

1981;

Poorman et

al, 1981;

Moses et

al,

1982;

Guichaoua et

_al,

_1985;

Gabriel-Robez et

al,

1986;

Batanian and

_Hulten,

1987)

but in others

_asynapsis

was followed

_directly

_by

_{heterosynapsis}

(Greenbaum

and

_Reed,

_1984;

_Kaelbling

and

_{Fechheimer, 1985; Hale, 1986;}

Hale and

_Greenbaum,

1988;

Gabriel-Rodez et

al, 1988;

and the

_present

_case).

There is no clear

_explanation

as to what factors are

_responsible

for the

_pairing

behavior of bivalents which are

_heterozygous

for inversion. In a review of 6 cases

of

pericentric

inversions in

_humans,

de

_Perdigo

et al

₍₁₉₈₉₎

indicated that

loop

formation occurred

_only

in those cases where both

breakage

_points

were located

within the

_G-light

bands. In other cases,

only heterosynapsis

occurred. This

who

_suggested

that

_pairing

behavior of chromosomal aberrations

_depended

_upon the

breakpoint positions.

In the

_present

case the breaks occurred in the

light

and dark

G-bands;

however,

it is also

_important

to notice that a

fairly

small chromosomal

fragment

was inverted. In such a _case, due to the shortness of the inverted

_fragment,

establishment of

_loop

formation may be

impossible

for

physical

reasons.

In the

_present

_study,

behavior of the sex bivalent was used as an indicator of

the sequence of

pairing

events

_{during pachytene.}

Additional association or

_pairing

between the free ends of the sex chromosomes has

already

been described in

_pigs

(Switonski

and

_Gustavsson,

_1986).

The same

_phenomenon

has also been observed in human

_{spermatocytes}

_(Chandley

et

_{al, 1984; Solari,}

_1988).

The latter author has shown that the

_frequency

of such a

configuration

increased

during

meiotic

_pairing

from the late

_{zygotene/early}

_pachytene

to later

_substages

of

_pachytene.

It seems

reasonable to assume that due to the so-called correction

_phase

of late

_pachytene

(von

Wettstein et

_al,

₁₉₈₄₎

association between free ends

_(type

₂₎

is followed

by pairing

between free ends

_{(type 3).}

The

_tangled

structure of the sex bivalent

(type 4)

is

_generally

classified as the last

_substage

of

_pachytene

and in Solari’s

(1988)

classification is called a &dquo;net-like filamentous

_{arrangement&dquo;. Taking}

into consideration the above-mentioned

_remarks,

it can be concluded that the observed

delayed

synapsis

in

_pair

No 8 occurred

_during

the first

_part

of

_pachytene

_(types

1 and

_2).

On the other

hand,

the low

_frequency

of

_{spermatocytes}

with

_delayed

pairing

seemed to be a result of the _very short

_length

of the inverted

_fragment.

This

_probably

caused a marked

_quickening

of the _process

_{of heterosynaptic &dquo;straight}

pairing&dquo; .

Depending

on the

_pairing

_behavior,

2 main

_types

of meiotic

_{segregation may}

be

expected. Loop

formation

_{establishes, by complete}

_homologous

_pairing,

conditions for

_crossing

over within the

_loop.

If this

_happens

then dicentric or acentric

chromosome

_fragments

will arise. In such a case the

fertility

of the inversion

carrier will be reduced due to the

_production

of unbalanced

_gametes.

Lack of

loop

formation and

_{subsequent heterologous pairing prohibits crossing}

over events

within the inverted

_fragment.

This causes

_regular

chromosome

_segregation

and the

fertility

of the carrier is not affected. The latter situation

_probably

occurred in the

present

case.

Unfortunately,

data

_concerning

the

fertility

of the carrier-boar was

not available.

It is known that

_unspecific

association between the sex bivalent and

autoso-mal

_{configuration, particularly}

_consisting

of

_rearranged

chromosomes

_(trivalents,

quadrivalents,

etc),

can cause activation of an X chromosome in

_primary

spermato-cytes,

which is

_responsible

for the arrest

_{of spermatogenesis}

_(Lifschytz

and

_Lindsley,

1972).

In the

_present

case, as well as in other cases of inversions described

_by

other

authors,

no associations between the sex bivalent and the autosomal bivalent

car-rying

an inversion were found.

Besides meiotic disturbances in carriers of chromosome inversions the effect of

position

should also be taken into consideration. The new _gene

_{neighbourhood}

at

breakage/rejoining

points

can also cause activation or inhibition of _gene

_expression.

ACKNOWLEDGMENT

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