DNA Polymorphisms of Casein Genes in Local Goats Population in the Southern Tunisia
Ouni Mabrouk ,
Gaddour Amor ,
The type and frequency of local goat milk casein αS1 and αS2 alleles was studied using about 150 individual milk and goat blood samples. The study starts the establishment of the local goat population genetic molecular characterization through the casein gene polymorphism. Genotypic and allelic individual DNA composition was realized by molecular analysis procedures. Results highlighted the local population large variability either for casein allele and variants. The most represented casein phenotype in the population was AB with about 38% as frequency; the BB and AA phenotypes have, respectively 36 and 17%. The genetic determinism of the local goat illustrates some specificity towards Mediterranean caprine breeds. Also, a large polymorphism seems to exist among local goat pigment types showing an intra population genetic diversity. For the allele A, the most variant identified was Ab. The high presence of the allele O within the local population (12%) distinguishes the indigenous population among caprine breeds. The traditional consanguine mating in the pastoral mode explains the O presence. The DNA polymorphisms of the caseins showed a high genetic variability and the possibility to select high productive local goats. Genetic improvement plans should account with the population specificities, especially at the level of the milk quality.
The local goat population represents an animal group raised since centuries in the Tunisian arid zone. The natural and human selection process oriented, during long period, this population to acquire a gene pool adapted towards range lands harsh environments and resources scarcities (Najari et al., 2007a; Gaddour et al., 2007a). Among the Tunisian national goat herd, more than 60% of livestock was actually raised in arid zone where pastoral breeding contributes to regional economy (Ouni et al., 2006a; Gaddour et al., 2007b). In fact, local goat remains as a rare genetic group able to valorise pastoral resources when other domestic species strive to survive (Najari, 2005; Najari and et al., 2007a).
The local goat pastoral herds have reduced production due, mainly, to the resources scarcities and the climatic negative impacts (Najari, 2005). Notwithstanding this relatively small overall impact, caprine productions play an essential part in certain difficult and marginal environments, where they often represent one of the rare sources of high quality proteins (Boyazoglua et al., 2001). The main local breeds' production, under extensive mode, is the kid's meat and milk is usually considered with secondary importance, especially in the herd cash flow (Barillet, 2007).
After a long marginalization period, goat production interest has recently increased in many countries (Morand- Fehr et al., 2001). In the Mediterranean zone, goat milk is either produced in intensive modes when specialized caprine breeds were used and also, in southern marginal zones by exploiting local breeds and populations (Boyazoglua et al., 2001).
Both for kids growing and for milk industrialisation, the milk production quantity and quality were determining. Milk protein composition differs quantitatively as well as qualitatively among goat breeds and genetic groups (Chilliard et al., 2006). The protein and fat contents variability is genetically controlled, especially by the as-casein locus, which exhibits a high degree of polymorphism (Ricordeau et al., 1999). Also, the casein polymorphism is related with various allele mutations (Martin, 1993). As for selected breeds, for an animal population a large variability is currently expected at all biological level due to the large genotypic diversity. So, understanding the polymorphism of caseins contributes to characterize every population genetic structure and evolution (Naves, 2003). Such knowledge can, equally, offers some precious improving ways for local populations especially when the milk quality and cheese output is considered as a selection criteria.
A stochastic approach is proposed to predict responses to selection when using αs1-casein genotype information in a selection scheme of a local goat's population when two independent selection objectives were considered: protein yield and protein content. Inclusion of the casein genotype as an additional selection criterion will improve gains for protein traits (Sánchez et al., 2005).
The gene of casein alpha-S1 presents many alleles. One can gather them in four categories according to their influence on the rate of casein alpha-S1 in milk: the alleles A, B and C are strong, the allele E is average, the alleles D and F are weak and the allele O is null. A polymorphism with multiple alleles was found in most Mediterranean breeds (Martin, 1993; Jordana, 1996; Moioli et al., 1998).
The Tunisian local goat population was subjected to a genetic characterization in the Arid Land Institute laboratories. Since a decade, pastoral husbandry and animal performances was studied to define local goat productive behaviour under arid conditions. Actually, many scientific results and parameters were established (Ouni, 2006; Ouni et al., 2006b; Najari, 2005; Najari et al., 2007a; Najari et al., 2007b; Gaddour et al., 2007a; Gaddour et al., 2007b ; Gaddour et al., 2007c). To diagnose deeply the local goat genetic characteristics, the study of the DNA polymorphism of some gene is started in the aim to valorise genetic specificities and to build selection criteria for our local population. Note that the type and frequency of goat milk protein alleles have not yet been studied for the local goats population. The present research reports the initial and partial characterization of allelic polymorphism of casein alpha s1 of local goat. Thus, the aim of the present study is to present the local goat polymorphism regarding the casein s1 as a step to characterize this rustic population under arid harsh environment.
MATERIALS AND METHODS
The experiment was carried out in arid region of Tunisia as ecologically
defined by Ferchichi (1996). The climate is arid Mediterranean, hard and precarious
(Ouled Belgacem, 2006). With an annual average of 140 mm, precipitation presents
a large spatial and seasonal variation. The arid zone is actually the most important
of rangelands of the country and is mainly used for extensive camel and small
ruminants grazing (Nasr et al., 2000).
Animal materialThe indigenous goat population constitutes an animal group adapted
to the arid rangelands harsh conditions (Najari et al., 2006). The complex
adaptation criteria were acquired by a long natural and human selection process
under local hard conditions (Najari, 2005). The indigenous goat population shows
a large variability both in morphology and performances (Najari, 2005).
|| Blood and milk samples repartition by morphologic characters
of local goat
Characteristics of the population include the ability to walk long distances,
water deprivation resistance and good kidding ability. Fertility rate is about
87% and prolificacy rate varies between 110 and 130% (Najari, 2005). Kidding
season begins in October and continues till February with a concentration in
November and December when 69.2% of kids are born.
Samples Collection and Analysis
The determination of the alleles casein α was limited to a subset
of 150 goats raised in pastoral mode in different regions of arid zone. Samples
collection and analysis were done during 2000. Table 1 presents
the data distribution by morphologic criteria. Samples were collected in three
herds raised in the Tunisian arid zone and the individual goat characteristics
were registered to diagnose possible relation between genetic polymorphism and
The frozen milk and blood samples were analyzed by electrophoresis to typify caseins αS1 and αS2 either at phenotypic and genotypic levels. Samples of fresh blood were treated, for extraction of the DNA, for different typing thanks to techniques developed at Institut National Recherche Agronomique (France) laboratory (Ricordeau et al., 1999).
RESULTS AND DISCUSSION
Genotypic Frequencies of the Casein Polymorphism for the Local Goats
Table 2 shows the phenotypic frequencies of milk α
S1casein polymorphism observed in the local goat population in Tunisian arid
zone. The phenotypes AB, B and A seem to be, respectively, the more represented
within the population. So, the milk quality can be considered high due to the
relation of these phenotypes with milk production quantity and quality in the
caprine breeds (Chilliard et al., 2006; Angulo et al., 1994).
Milk from homozygote phenotype A goats has higher protein, casein and fat contents
and a higher casein/protein ratio than milk from FF goats (Vassal et al.,
1994; Delacroix-Buchet et al., 2000). So, the local population genetic
profile is specific and it illustrates, in parts, its dairy characteristics
towards other caprine breeds and populations.
In other respects, the genetic polymorphism at the alpha-s1 casein locus has important effects on the true protein, the casein and the fat content of milk, the maximum gel strength and the gel strengthening rate (Remeuf, 1993).
Rather than this local population specificity, important intra population variability
is show on Table 2. This large variability confirms the marked
differences between breeds and also intra populations and goat genetic groups
(Moatsou et al., 2004; Najari, 2005). The large caprine variability,
concerning the casein polymorphism, was observed in the Mediterranean area and
within the same regions and countries (Enne et al., 1997; Grosclaude
and Martin, 1997). Hence, among our indigenous pigment types, a large variability
of phenotypes frequencies exists in studied morphological goat types.
|| Frequencies at the S1casein genotypes for 150 samples of
local goat population
|| Genotypic frequencies of the casein αS2 with respect
to the local goat pigment type
All phenotypes vary largely from a goat pigment to another (Table
1). This local population large variability, regarding casein types, collaborates
with the large morphologic and productive variability intra population for local
goat concluded by several authors (Najari, 2005; Najari et al., 2006).
For the black local goat, which is the principal pigment type present in our population (Najari, 2005), the most present casein phenotype is the AB. However, the phenotype A is not observed for the pigment light belly.
AA homozygous genotype, which represents about 17% in our population (Table 2), is related with lower milk yield compared to AE, AF, EE and FF goat genotypes (Moioli et al., 1998). This can explain the low local population milk production which estimated to 97 kg per lactation in average (Najari, 2005). However, the homozygote AA were related to high protein percentage. Moioli et al., (1998), assigned for AA a genotype an average of 3.5 g protein/kg milk for Murciana-gradina breed. Milk fat percentage is also higher in AA and AE confirming, thus, the particular local goat milk quality.
Table 3 presents the frequencies of the casein alpha s2 in
the local population and its variation with respect to goat pigment type. Results
show that the most represented milk phenotype is the AA and AC with, respectively
54 and 37%. The phenotypic variability seems to be more restricted than the
variability of casein alpha s1. As well as for casein alpha s2, the genotypic
polymorphism at casein alpha s2 affects significantly the milk production and
composition (Plan de développement de la R-D, 2002-2007) With respect
to goat pigment types, Table 2 shows the phenotypic frequencies
of genotypes AA and AC for different goat morphological types. Note that some
goat pigment types have more equilibrated genetic profile than others. The polymorphism
variability at casein alpha s2 is shown between caprine breeds, populations
and intra populations (Marie-Michèle Filion, 2006).
Allelic and Variants Frequency of the Casein Alpha S1 for the Local Goats
Table 4-6 show the frequencies of alleles
in the local population and by goat pigment type. Frequenciesy of allele B3
and B4 were 43 and 57%, respectively in local population. The allele A is considered
as favourable with its contribution in the hypophysarian secretion which affects
the dairy performances by the bias of the GH, PRL and TSH-B synthesis (Gengler
et al., 2001). More ever, the same author indicated that the A allele
presence can be related with the increase of the total milk production.
|| Alleles frequencies of the casein αS1 for the local
|| Frequencies of allele A variant in local goat population
|| Frequencies of all casein allele variants in local goat population
Mean micellar size is lower in A milks (Pierre et al., 1998b; Remeuf,
1993). Lipolysis is lower in A milk and the relative proportions of FFA were
different in A and O or F milks (Lamberet et al., 1996; Pierre et
al., 1998a). Total solids, fat recovery and gross yield are significantly
higher in cheese made from milks of goats with high rate of as1-casein synthesis
(Remeuf, 1993; Vassal et al., 1994).
Cheeses made from FF milks exhibit a more intense goat flavour than that made from AA milks (Delacroix-Buchet et al., 1996; Lamberet et al., 1996; Vassal et al., 1994). Also, caprine milk, with very low as1-casein content, was found to be less allergenic than that with a high level of as1-casein (Bevilacqua et al., 2001). The B allele controls the cheese quantity and the coagulation time (Gengler et al., 2001). The high frequency of the allele B in the local population (Table 4) illustrates a relation with the adaptative behaviour of the indigenous goat and the quality of its milk.
The allele O presence in our population seems to be high with respect to other Mediterranean caprine breeds and populations (Moatsou et al., 2004). According to Chianese et al. (1993), allele O in goats is associated with very low casein content in milk. The presence of the allele O in the local goat population illustrates a consanguine mating traditionally adopted in the pastoral mode. In fact, in the pastoral breeding mode of the local goat, the bucks were mainly selected in the same herd progeny and they were kept during a long reproductive period (Najari, 2005; Najari et al., 2002). The consanguine mating, characterizing local goat husbandry, can explain the large genetic variability observed between genetic groups intra population.
The casein allele repartition according to the goat pigment shows the genetic variability of this population for the production of the milk in the arid zone is (Table 4). It is noticed that the alleles expressed differently for the various goats pigments especially for the allele B witch vary form 2% for the pigment pinto to 13% for the goat black.
For each casein allele, the variants frequencies were presented on Table
6. Regarding the allele O, the variant O3 is the more present in the local
goat population. Four variants of the allele B were present in our indigenous
population; these four alleles were described by Ricordeau et al. (1999).
The quantitative effects of goat αs1-casein variants on dairy traits were
studied in both on-station (Barbieri et al., 1995) and on-farm experiments
(Mahe et al., 1993).
The study of the casein polymorphism allowed to characterize the local goat population and to illustrate this genetic variability at the level of a dairy character. This study adds an important knowledge about the population structure and it can help to understand its genetic evolution. The polymorphism at the casein locus is now sufficiently documented as well as its relation with the population husbandry and consanguine mating.
Allelic frequencies illustrate the local population large genetic variability and its' dairy production characteristics genetically acquired through natural selection under arid harsh environment in the southern Tunisia. Also, some specificity towards Mediterranean breeds was detected. This can help to apprehend our local goat diversity studying and capacities valorisation. The large genetic population variability, at the casein polymorphism, contributes to identify some animal group able to select dairy local goat. A particular importance has to be allowed to the milk quality study in the aim to valorise local goat milk production. In fact, results underlined that, even though reduced, local goat milk production has genetically a determinism which supports protein high quality.
In spite of the results importance, they remain only a first step to characterize the local population genetic structure and capacities. In fact, scientific priorities should be directed firstly towards homogenous groups' identification and the study of the variants frequencies within the population sub groups. Further more, is necessary to determine if we can increase the local goat milk production and components without deteriorating qualities which differentiate indigenous goat's milk from other caprine selected breeds. Also, it will be necessary to found genetic markers associated proteins other than caseins and which would have an effect on quality of caprine milk.
Then, proteinic polymorphism describes only a part of DNA polymorphism reality and to evaluate genetic diversity, it necessary to identify some markers who describes better the whole genome polymorphism of the genome, that is to say microsatellites.
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