Selection on testis size as an indicator of maturity in growing animals. I Direct and correlated responses in growth

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Selection on testis size as an indicator of maturity in

growing animals. I Direct and correlated responses in

growth

Wg Hill, Pj Marks, Jc Jenkins, Rb Land

To cite this version:

Original

article

Selection

on

testis size

as an

indicator

of

_maturity

in

_growing

animals.

I

Direct and correlated

_responses

in

_growth

WG

Hill

1

PJ Marks

JC Jenkins

1

RB Land

2

1

Institute

_{of Animal Genetics, University of Edinburgh,}

West Mains- Road,

Edinburgh

EH9 _3JN;

2

AFRC

Institute

_of

Animal

_Physiology

and Genetics _Research,

Edinburgh

Research

_Station,

_Roslin, Midlothian EH25

_9PS,

UK*

(Received

2 October _{1989; accepted} 15 _February

₁₉₉₀₎

Summary - Selection was undertaken in mice to test whether

_gonad

_size,

_specifically

testis

weight,

could be used to indicate

_degree

of _maturity and _produce

_{genetic change}

in the

shape

of the

_growth

curve from records on _young animals _only. Selection was _practised

for 11

_{generations high}

and low on 5 wk male

_{body weight}

_(lines

_HX,

_LX),

on 5 wk testis

weight

(XH, XL)

and on indices with these traits selected in the same

(HH,

LL)

and in

opposite

(HL,

LH)

directions. There were 2 _replicates of each selection line and 4 unselected controls.

_Divergences

between

_high

and low lines within

_family

standard deviation units for 5 wk _body

_weight

and testis

_{weight, respectively,}

were: HX-LX: 1.8 and _2.3, XH-XL: 2.2 and _4.2, HH-LL: 1.9 and _3.4, and HL-LH, 2.0 and -2.7. There were minor _changes

in _shape of the _growth curve in the direction _expected, with those selected for

_high

testis

weight showing

earlier _maturity. A _simple measure, 6 wk

weight/15

wk

weight

(%),

for

entire males was: HX _74.3, LX _67.3, XH _71.8, XL _69.6, HH _78.1, LL _72.6, HL _74.2, LH _78.5, averaging 76.1 for lines selected for _high testis _weight and 72.1 for _{low, compared} to 74.0

for controls. In castrate males, a similar pattern of

_changes

in

_growth

curve was observed

indicating

that _they were not a consequence of testis derived hormones. Mice selected for

high

testis

_weight

were leaner than those selected for low testis

_weight.

_Thus,

_growth

curves

changed

due to selection on testis

_weight,

but it was not established if this is because testis size is an indicator of sexual _maturity.

growth / maturity / testis / selection / mouse

Résumé - Sélection par la taille des testicules considérée comme un indicateur de la maturité des animaux en croissance. 1 Réponse directe et _réponse corrélée sur les caractères de croissance. Une

_expérience

de sélection a été réalisée chez la souris

pour déterminer si la taille des _gonades, en _particulier la masse des _{testicules, pouvait}

indiquer le

_degré

de maturité et _{pouvait modifier} l’allure de la courbe de _croissance, à partir des seules données recueillies chez de _jeunes animaux. La sélection a été conduite

pendant 11

_{générations,}

vers le haut

(H)

et vers le bas

(L)

selon

plusieurs

critères: masse

*

corporelle chez le mâle à 5 semaines

_(lignées

HX,

LX);

masse testiculaire à 5 semaines

(XH,

XL);

indices combinant ces deux caractères, dans le même sens

(HH, LL)

ou en sens _opposés

_{(HL, LH).}

_{Chaque lignée} sélectionnée a été _répétée deux

fois,

_{et .¢ lignées}

témoins non sélectionnées ont été élevées. Mesurées en unités d’écart _type

_{intrafamille,}

les

_divergences

observées entre _lignées haute et basse ont été les _{suivantes, respectivement}

pour la masse _corporelle et la taille des testicules: HX-LX: _1,8 et _2,3; XH-XL: _2,2 et _4,2;

HH-LL: 1,9 et _3,l; HL-LH: _2,0 et _-2,7. On a observé de _petites

différences

de la

forme

des courbes de croissances dans le sens

_attendu,

les

_lignées

sélectionnées _pour une masse

testiculaire élevée montrant une maturité

_plus

_précoce. Le _rapport de la masse

_corporelle

à 6 semaines sur celle à 15 semaines était chez les mâles entiers

(en

pourcentage).’

HX:

7/,3; LX: _67,3; XH: _71,8; XL: 69,6; HH: _78,1; LL: _72,6; HL: _7.¢,2; LH: _78,5; avec une

moyenne de 76,1 chez les

_lignées

sélectionnées pour une masse testiculaire _élevée, contre

72,1 chez celles sélectionnées _pour une masse testiculaire

_faible,

et _74,0 chez les

lignées

témoins. L’observation chez les mâles castrés de

_{modifications}

similaires des courbes de

croissance

_{indique qu’elles}

ne _{sont pas} dues aux hormones sécrétées _par les testicules. Les

souris des

_lignées

sélectionnées pour une masse testiculaire élevée étaient

_{plus maigres}

que

celles sélectionnées pour une masse testiculaire

_faible.

Des

_{modifications}

des courbes de

croissance induites par la sélection sur la masse testiculaire ont ainsi pu être observées, mais on n’a pas établi que c’était parce la masse testiculaire est un indicateur

_spécifique

de la maturité sexuelle.

croissance _/ maturité _/ testicule _/ sélection _/ souris

INTRODUCTION

The animal breeders’ aim is to select animals of

_{high early growth}

but low mature

size so as to reduce costs of both the

_slaughtered

animal and its dam. Bone

_growth

of animals slows or terminates after

_puberty

under the influence of

_gonadal

_hormones,

so the

_object

of the

_experiment

was to test whether selection on

body weight

and

testis

_weight

on the immature animal could be used to influence both

_{early gains}

and mature

_weights.

This follows results in

_sheep,

where selection on testis

weight

of rams with

phenotypic

correction for

_body

_weight

led to correlated

_changes

in

mature

_weights

in their female

_relatives,

_high

testis

_{weight being}

associated

with

low

_{body weight,}

and vice versa

(Land

et

_al,

_1980;

Lee and

_{Land, 1985; Haley}

et

al,

1989

_(submitted);

Lee and

_Haley,

_1990).

The _present

_approach

differs from the more direct method of

_selecting

_directly

on

weight

at _{different ages which has been}

_applied

with

_varying

success in different

species.

Substantial

_bending

of the

_growth

curve has been achieved in both

turkeys

(Abplanalp

et

_al,

₁₉₆₃₎

and chickens

_{(Ricard, 1975).}

In

_mice,

_bending

has been

effected,

but to a lesser extent than in

_{poultry, by}

_McCarthy

and Doolittle

_(1977),

who selected on 5 and 10 wk

_weight

with the

_objective,

inter

_alia,

of

_increasing

5

and

_holding

10 wk

_weight

_constant,

_by

Wilson

₍₁₉₇₃₎

who selected on the ratio of

3-6 wk

_gain

to 3-9 wk

_gain,

and

_by

von _Butler et al

_{(1980, 1986)}

_using

3-5 wk

_gain

and 8 wk

_weight.

These

_{schemes, however,}

_require

selection decisions to be deferred

until

_animals

_approach

their mature

_weight

which is after _{the usual age} of

_slaughter

and

often

after the _age of first

_mating,

when

_culling

decisions have to be made in

both mice and domestic livestock.

In the present

experiment,

mice were selected

_high

and low for

_body

and testis

weight

on either trait alone or on indices of the 2 traits in either the same or the

size may also be an indicator of ovulation rate and litter size

_{(Land 1973),}

records

of

_{reproductive performance}

were

kept

and will be

reported

in a

subsequent

paper

(Hill

et

_al,

_1990).

Further details of the

_experiment

are

reported

in PhD theses

by

Jenkins

_(n6e

_Williams,

₁₉₈₄₎

and Marks

_(1988).

MATERIAL AND METHODS

Mouse stocks and

_housing

The mice for

_generation

0 of this

_experiment

were crosses at

_generation

3 or 4 between

_pairs

of the unselected control lines of the G strain established from a

3-way

cross of 2 inbred and 1 outbred strain

_(Sharp

et

_al,

_1984).

Four _separate sets

of crosses were made to establish the different foundation groups. The mouse house

was maintained at 21 f 1°

_C,

RH

_30-40%,

with water and BP’s rat and mouse No

1

_expanded

maintenance diet

_provided

ad libitum. Selection lines

Twenty

lines were established as shown in table

_I,

with 2

_replicates

of each of 8 selected lines and 4

_replicates

of the unselected controls. The lines were

_managed

as

4

_contemporary

_groups

_spaced

at 3 wk

_intervals,

each _group

_comprising

4 selected lines

_(which

were either the 4

_single

trait lines or the 4 index

_lines)

and 1 control.

Mice were mated at

_{approximately}

9 wk of _age.

_Eight

_pair

_matings

_per line were

made each

_{generation, using}

the maximum avoidance scheme of Falconer

_(1973),

together

with 4 reserve

_matings

which were

_only

used if _necessary. At

_birth,

litter

of males was

practised,

while females were taken at random within

_families,

as were

males in the controls. The selection

_phase

of the

_experiment

was continued for 12

generations,

but no selection was

practised

in

_generation

7.

A

_{preliminary study}

showed that 5 wk of _age was an

_appropriate

_age for selection. Both

_body

and testis are then

_{growing rapidly}

and there is a

_high

(0.99)

correlation of

_right

and left testis

_weight

to allow hemicastration. The coefficients of variation

of

_body

_weight

and testis

_weight

were then 0.17 and 0.26

_respectively

and their

phenotypic

correlation was 0.50.

Body

weight

at 5 wk was recorded on all males. Testis

_weight

at 5 wk was

recorded every

generation

in lines selected on testis

_weight,

but not every

generation

in the others

_(ie

_{HX, LX}

and

_CC).

To do

_this,

mice were anaesthetised

_by

Sagatal

and

_{hemicastrated,}

and 1 testis was excised and

_weighed.

The indices used were

7 == ± t!/o-! ± T/o-r = ! 0.54 tV ± 0.112 T

where W and T denote

_{body weight}

_(g)

and testis

_weight

_(mg);

and Uw and er

r denote their within

family

phenotypic

standard deviations in

generation

0,

estimated at 1.82 _g and 8.9 _mg

_{respectively.}

In line

_HL,

for

_example,

mice were

selected for

_high

values of I = 0.54 W — 0.112 T each

_generation.

Data recorded

Analyses

of

_growth

curves were made at

_generations

7 and

_13,

after 6 and 11

generations

of

_{selection, respectively.}

At

_generation

_13,

litters were standardized

to 7 _young at

_birth,

and at 19 d half the male mice were castrated and

_dummy

operations

were undertaken on the rest.

_Eight

entire and 8 castrated mice of each line were

_weighed,

at least

_weekly

from

_weaning

at 3 wk until

_they

were 17 wk old. Food intake was recorded at

_generation

13 on the same

_mice,

who were

_caged

in

pairs

and fed ad libitum. Gonadal fat

_pads

and hind limb fat

_pads

were dissected on

11 wk old entire males of

_generation

_11,

and chemical

_analysis

of the whole minced carcass was undertaken on the mice of

_generation

13.

Statistical

_analysis

The

_experiment

was

_basically

a

_partial

block

_design,

with each block

_comprising

5

lines,

each line

_replicated

over 2 blocks.

_Apart

from the block

_(group)

effects which

were taken as

fixed,

there were 2 other sources of error associated with a line mean:

genetic

drift accumulated over the selection and error due to

_recording

limited numbers. This combined error variance was estimated as the interaction variance

between selection line and

_{replicate. Thus,}

the model used for observations taken

only

on males was

where:

_Yg

_hij

is an observation on

_{individual j}

in

_replicate

h of line i within the

type of selection _g; u is the overall _mean;

_Tg

is the effect of _type of selection

_(g

= ₁

for

_{single trait, g}

= 2 for

index;

Rg

h

(h

=

1, 2)

is the effect of the hth

replicate

nested within the

_gth

type of

_selection;

_Lg

_i

_(i

=

1, ... , 5)

is the effect of the line

although

with

_only

8

_df;

_{and e9hz!} is the

_remainder,

which in some

_analyses

was

partitioned

to include a litter effect.

RESULTS ‘

Responses

in 5 week

_body

_weight

and testis

_weight

Generation means for

_body

_weight

and testis

_weight

of 5 wk old males are shown

in

_figures

1-5. For

_clarity,

results for

_{pooled replicates}

are shown in the

_graphs,

but

in table

_II,

differences between

_pairs

of selected lines at the end of selection

_(means

of

_{generations 10,}

11 and

₁₂₎

are shown for each

_replicate.

In the

_single

trait lines there were direct responses

(as

divergence

between

high

and low

_lines)

of the order of 2 sd

_(within

_family

standard

_deviations)

for

_body

weight

and 4 sd for testis

_weight,

and similar

positive

correlated responses of about 2 sd in testis

_weight

and

_{body weight respectively. Thus,}

there were substantial

positive

correlated _{responses in} index

_1,

where

_{body weight}

and testis

_weight

have the same

_sign,

and

_negative

correlated responses in index

2,

albeit very small in

the

_{body weight}

lines. There _was,

_however,

substantial variation between

_replicates

(table II),

and,

although

selection differentials were similar in

high

and low

lines,

some _asymmetry of response

(figs 1-5).

The

replicate

lines

_{comprised only}

8

_mating

pairs

and so between

_replicate

and

_direction,

variation would be

_expected

from

genetic

sampling.

The

_analyses

have

_therefore,

to minimise their

_number,

been

based on

_divergence

between

_high

and low lines.

There were _responses of the same order in the index 1

lines,

ie about 3 sd in the

index

_itself,

with

_positive

correlated responses of 2 and over 3 sd in

_{body weight}

and testis

_weight

_{respectively,}

and

_thus,

since testis

_weight

had

_{changed relatively}

single

trait selection indicate a

positive

_genetic

correlation between the

_traits,

with index 2 there was a substantial direct response of 4 sd in the index and correlated

responses of the order of 2 sd of

increasing

body

weight

and

decreasing

testis

weight.

Estimates of realized

_heritability

from within

_family

selection

_(h!)

obtained from

regression

of

_divergence

of response

( eg HX-LX)

on cumulative within

_family

section

differential each

_generation

are

_given

in table III.

_Using

the intra-class correlation estimate

_(t)

from the base

_population,

estimates of

_heritability

were obtained as

h

2

=

2(1-

t)hw

(Falconer, 1989).

Both

hw

and h

2

estimates _were

higher

for testis

weight

and index

_2,

where

_antagonistic

selection was

_practised,

than for

_body

weight

and index 1.

_Heritability

estimates were lower than

previously

obtained for

body weight

in this

_population

_(Sharp

et

_al,

₁₉₈₄₎

and testis

_weight

on a different

From the

_pairs

of

_single

trait selection lines an estimate of

_genetic

correlation is

_readily

obtained as

w

l/2

)J

R

T

)/(R

w

C

T

[(C

_where,

for

_example,

_R

_w

_,C

_T

denote the direct response in

body

weight

and correlated _{response in} testis

_weight

from selection on

_body

_weight

_{(Falconer, 1989).}

From table

_II,

the estimates of correlation

are

0.85,0.49

and 0.80 from

_replicates

_1,

2 and

pooled, respectively.

An estimate of

the

_genetic

correlation from the index lines can also be

_obtained,

and the method of

calculation is

_given

in the

_Appendix.

This estimate was 0.17 for

_replicates

_pooled.

It is clear that the index selection was more effective in

_{producing divergence}

of

heritability

of index 2 than of index 1 would be if the environmental correlation were much

_higher

than the

_genetic

correlation.

Analysis

of

_growth

The selected lines did not differ

_greatly

from each other or from the controls in the

general shape

of their

_growth

curves at

_generation

13. There were however some

these.

_Therefore,

in order to show the

_differences,

mean

_weights

of males of the selected lines are

_given

in

figures

6 and 7 for entire and castrate

_males,

_{respectively,}

as a _percentage of the

_weights

of the

_{corresponding}

controls at the same _age. The control means are

_given

in table IV. For

_example,

both the entire and castrate males of the LL index line are about 10% smaller than those of its control at 21

_d,

but

_only

2% smaller at 119

_d;

whereas those of the LH line are also smaller than the

_controls,

but

_relatively

less so at 21 than 119 d. Even at 17 wk when

_recording

_ceased,

mice

An alternative to

_expressing

_{body weights}

of 1 line as a ratio of those in another

is to use

_logarithms,

and

_particularly

natural

_logs

because when ratios

_y/x)

are near

_unity,

_{y/x -1 ;zt; In y -in x.}

Some contrasts between relevant

_pairs

of lines are

given

in table IV

_(scaled

up x 100 to remove

leading

decimal

zeros),

and some data are included from the

study

of

body weight

taken at

_generation

_7,

_midway

_through

the

_experiment.

_Apart

from 6 wk

_(strictly

41

_d)

when LX mice were

_atypically

small,

the

_divergence

in

_{body weight}

between lines HX and

_LX,

selected for

_body

weight,

was similar at different _{ages in} the entire

_males,

but tended to increase in

the castrates. The

_divergence

in

_body

_weight

between XH and

_XL,

the testis

_weight

selected

_{lines, consistently}

decreased with _age, as was the case between the index

lines HH and

_LL,

most

_notably

at

_{generation 13;}

these results could be

_expected

if

_high

testis

_weight

lines matured earlier. In _contrast, the

_divergence

between the other index

_lines,

HL and

LH,

remained more _constant,

_tending

to increase with

estimated from differences between

_replicates

with 8

_degrees

of

_{freedom, only}

a small number of

_comparisons

of

_weights

were

_significant.

Relative

_maturity

was

computed

as the ratio of

body weight

at 6 wk to that at

15 wk for data from both

_generations

7 and 13.

_(Records

ceased at 15 w in

_generation

7. In

_generation

13,

records at 41 and 107 d were

used).

At 6 wk the mice had achieved about 70% of their 15-wk

_weight

_{(table V).}

The lines selected for

_high

testis

_weight,

ie

_{XH, HH}

and

_LH,

were all

_relatively

earlier

_maturing

than the

corresponding

lines selected for low testis

_weight,

ie

_{XL, LL}

and

_HL,

the difference

being

about

4%

on _average.

Analyses

of the records on food intake taken in

_generation

13 showed that the _{responses in}

_gain

were

_generally

_accompanied

_by

similar

_proportional

_changes

in food

_intake,

so there were

_only

trivial differences between the lines in food conversion

_efficiency

_(results

not

_shown).

Body composition

Body

composition

was assessed from the ratio of fat

pad

_weights

(gonadal

plus

hind limb

_depots)

to

_{body weight}

on entire males of 11 wk of _{age in}

_generation

10,

and from total chemical fat as a

_proportion

of

_(wet)

_body

_weight

on entire and

castrate males of 5 and 17 wk at

_generation

13

_(table

_VI).

The lines differed little

in fat content at 5 wk. At _{later ages there} was no very consistent pattern among

those selected on

_{body weight,}

but the lines selected for

_high

testis

_weight

were,

DISCUSSION AND CONCLUSION

The

_objective

of this

_experiment

was to check the

_hypothesis

which arose from

the

_experiment

with

_sheep

_(Land

et

_al,

₁₉₈₀₎

that the _pattern of

_growth

could be altered

_{by using}

a measure of

_maturity

such as testis size on immature animals.

Undoubtedly

such a

change

was

effected,

perhaps

seen most

_{clearly by}

the ratio of 6-15 wks in table V for the mean

_comparisons

of

_maturity

of

_high

and low testis

weight lines,

which differed

_by

about 4% around a mean of 70%.

_{Nevertheless,}

the results were not

_{&dquo;spectacular&dquo;: they}

are not

_clearly

seen in

simple plots

of

body

weight against

age, so the derived results shown in

figures

6-7 were necessary.

One of the more

_striking

_aspects of the

_growth

curve

_analyses

is that very similar

changes

were effected in both entire and castrated males

_(see

table

_V),

and

_analyses

(Williams,

1984,

not

_tabulated)

at

_generation

7

_yielded

similar results for females as for males. This

_implies

that the _presence of the testis after 19

_d,

the _age of castration

at

_generation

_13,

and

_circulating

testosterone are not

_required

for the

_growth

curves

to

_change.

Estimates of testosterone levels were made on 5 and 10-wk-old mice

by

the method of radioimmune _assay of Webb et al

_(1985).

Differences between lines were,

however,

small

compared

to error from natural and assay

variation,

for

correlations of line means on successive

_samplings

were low

(<

0.5),

and no clear differences were found

_(results

not

_shown).

In

_addition,

_age at sexual

_maturity

of males was assessed in

_replicate

1

(there

was not time to

_repeat

in

_replicate

₂₎

the

_day

of first

_{copulatory plug.}

The control mean was 43.5

d,

and the contrasts

were HX-LX =

_-2.0,

XH-XL =

_4.4,

HH-LL =

_-1.2,

HL-LH =

_4.7,

each with SE

of 1.1

_including

_only

within

_replicate

error. In _summary, the mean for

_high

testis

weight

lines was 0.5 d less than for low testis

_{weight lines,}

a trivial difference

_(a

few

mice did not

_plug

_by

49

_d,

when

_{recording ceased,}

but

_accounting

for them would

not influence this

_conclusion).

It is therefore unclear from this

_experiment

that the

_change

in testis

_weight

induced

_changes

in

_growth

rate as a

specific

indicator of

sexual

_maturity

or

_{directly through}

its hormonal action.

_{Alternatively,}

testis

_weight

in relation to

_body

_{weight might}

_simply

be an indicator of allometric

growth

of internal _organs, and that

_high

ratios of

_weights

of several _organs to

_{body weight}

are an indicator of

_approaching

_maturity.

In the

_experiment

with

_sheep

_(Land

et

_al,

_1980),

the selection criterion was testis

weight averaged

over _{3 ages,}

_6,

10 and 14

wk,

adjusted by phenotypic regression

on

body

weight

at _{these ages}

_(Haley

et

_al,

_1989).

Thus

_they

were, in

effect,

selected on an index of

_high

testis

_weight

and low

_{body weight}

at these _ages and the

_changes

in

_{body weight}

could

_largely

be due to selection on

_{body weight. Haley}

et al

₍₁₉₉₀₎

calculate

_however,

that the

_genetic

correlation between their selection criterion in

males and

_{body weight}

in females became

_{increasingly negative}

with successive years of age of the

female,

whereas

simple negative

selection on

body weight might

be

_expected

to

_give

a different trend.

Correlated responses in

reproductive performance

are described in a

_subsequent

paper

(Hill

et

_al,

_1990),

but these are relevant to the discussion of what influences

mature size.

_Briefly,

_positive

correlated responses in litter size were obtained with

selection on

_{body weight}

_alone,

testis

_weight

_alone,

and on the index of the traits in the same direction

_(HH-LL),

but not when selected in

_opposite

directions

_(HL-LH).

The

_analyses

undertaken on those results indicate that selection on testis

_weight

influenced litter

_size,

even after correction for

_{body weight,}

in

accordance

with the results of Eisen and Johnson

₍₁₉₈₁₎

and

_suggesting

that testis size is indeed an indicator of

_development

in some

_specific

_way, as Land

₍₁₉₇₃₎

_proposed.

From the results of this

_experiment

it is not

_possible

to

_directly

_compare the

accuracy of selection

including

records on testis

_weight

of immature animals to that

including body weight

on mature

_animals,

without estimates of

_genetic

variances and covariances of

_weights

_except at 5 wks. If the intention were to increase an

_early

weight

and hold a later

_weight

_constant, the response would then be a little less than

(1-rG)

1/z

of that with selection for the

_{early weight alone,}

where rG is the

genetic

correlation. For 5 and 10 wk

_body

_{weight, Hayes}

and

_McCarthy

₍₁₉₇₆₎

obtained

r

G =

_0.8,

for which

(1 &mdash;

T

&)

1/2

= 0.6.

_{Nevertheless,}

lines selected _on testis

_weight

and on the indices of 5 wk

_body

_weight

and testis

_weight

_gave similar _{responses in}

5 wk

_{body weight}

as did the lines selected

_solely

for that trait and lower but not

zero responses at later ages. Five-wk

body weight

had a rather low

_{heritability,}

both

to within

_family

selection and also when converted to mass

_{selection, substantially}

lower than for testis

_weight;

therefore testis

_weight

was a better

_predictor

of

_body

weight

than vice versa. This may not

apply

to other

_species.

The index of

_practical

interest,

ie

_increasing

_body

_weight

and testis

_weight

_(HH),

_similarly

had a low

heritability,

particularly

for mass selection because the intra-class correlation of sibs

in the correlated responses of the index lines and apparent

discrepancies

between

genetic

correlations of

_body

_weight

and testis

_weight

among the sets of lines.

Whilst there was

_reasonably

_good

evidence to

_suggest

that mature

_weights

were

reduced

_by

selection on testis

_weight

relative to

_body

_{weight, indicating}

that such animals matured

_earlier,

the

_fairly

clear observation that

_they

were also leaner was rather

_{surprising because,}

in farm

animals,

early maturity

is

_usually

associated with

higher degrees

of fatness when

_comparisons

are made at constant age. There is no evidence that the

_high

testis

_weight

selection led to _{any restriction} of

_appetite

which

might

have effected

_this;

nor _{is there any} obvious mechanism.

In

conclusion selection on testis

_weight

led to some

_changes

in the

_shape

of the

growth

curve. That these were not marked is not

_surprising

in view of the

_high

correlation between

_weights

at different _ages. At this

_stage

we would _not,

_however,

advocate selection to be

_practised

on testis

_weight

to

_change

_degree

of

_maturity.

It tis

_likely

to be safer to select

_directly

on

_weights

at different _ages; and if selection decisions need to be made

_early

in the animal’s

_life,

records of older relatives such

as _parents and uncles can be used.

ACKNOWLEDGMENTS

This work was

supported by

a

_grant

from the

Agricultural

and Food Research Council. We are _very

grateful

to Roberta

_Wallace,

Ann Walker and staff of the mouse house for technical assistance and to Gene

_Eisen,

_Douglas

Falconer and Chris

Haley

for comments on a draft of the paper.

Roger

Land initiated the

_experiment

reported

here and died after lab work had been

_completed.

He is

_greatly

missed.

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Ogasawara

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Asmundson VS

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71-82

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Pirchner F

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traits in

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₍₁₉₈₉₎

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APPENDIX

Estimates of

_genetic

correlation

estimates of

_genetic

variances

_(V

_A

_),

_genetic

covariances

_(Cov

_A

₎

and

_genetic

cor-relations

_(r

_A

₎

were

obtained,

where W =

body weight

(g)

and T = testis

weight

(mg),

and the units of the indices are

_arbitrary.

From the

_single

trait

_{lines, V,qyy}

=

2

(

0

.

26

)(

1

.

82

)

2

= 1.72 and

V

AT

=

₇₆

_,

and thus

Cov

AWT

=

(C

T/

Rw)V

AW

= 10.1 _or

C

OV

,g

WT

=

(CW !RT)vAT

=

8.2,

with a mean of

_9.1,

_{corresponding}

to r AWT =

0.80,

as in the text. From the index

_lines,

V

AIl

= 1.69 and

V

A12

=

1.24 and thus

COV

AIII2

=

(C

I2/

R

Il

)V

AIl

= -0.52 _Or

_OVAI112

_C

=

(C

Il/

R

I2

)V

AI2

=

_W

_.18,

with mean

_-0.35,

_giving

rAIlI2 = -0.24.

Predictions of index variances and covariances from the

_single

trait lines and vice

vers d

can also be made.

Letting

b

l

= 0.54

and b

2

= 0.112 be the index

weights,

then:

Y

All

=

b2V

AW

₊

2bib2 COV

AWT

₊

b22VAT, VA12 blV

-

AW

2b

1

b

2

COV

AWT

₊

b2V

AT

and

_Cov

_AI1I2

=

b1 V

AW

-

b’

2

V

AT

and

thus,

for

example,

V

AW

=

(V

A

n

₊

V

A12

₊

2

_{COV,q1112)!(4b1)·}

_Predicting

from the

_single

trait lines: VAn =

_{2.56, V}

_Arz

=

0.35,

CO

VAIlI2

= -0.45 and _rAll12

_{= 0.48;}

and

_predicting

from the index

lines,

using

the mean estimate of

Cov

All

1

2

= -0.35 :

_V

_AW

=

_{1.90, V}

_AT

=

73,

C

OVAWT

=

1.90

_{corresponding}

to rAwT =

0.17,

which _contrasts with the estimate of 0.80 from