ГЕНЕТИКА, 2010, том 46, № 8, с. 1132-1138
ГЕНЕТИКА ЧЕЛОВЕКА
УДК 575:599.9
DEMOGRAPHIC STRUCTURE AND MICROEVOLUTION OF AN ITALIAN ALPINE ISOLATED POPULATION
© 2010 G. Sella, A. Marin, A. Santovito, M. Girotti, P. Cervella, M. Delpero
Dipartimento di Biologia Animale e dell'Uomo, 10123 Torino, Italy e-mail: alfredo.santovito@unito.it Received September 23, 2009
The isonymic method has been generally accepted in population genetic studies and surnames have been successfully used to investigate human populations as if they were genetic markers associated to the Y chromosome. In this study we analysed the microevolution dynamics of Postua, a mountain village of North Western Italy, by means of demographic methods. The uniqueness of this community is due to its past geographical and cultural isolation and to the high frequency of marriage between relatives. During the study period considered (1640—1989) the population underwent several fluctuations in size and other demographic parameters such as the endogamy, isonimy and consanguinity rates. Until the first half of the XIX century Postua appears to be a village characterised by a closed and isolated community, with high endogamy (80—90%) and isonimy (up to 34.4%). Only at the beginning of the XX century, when the population started to be subjected to significant immigration fluxes, data show a progressive reduction of the isolation. The population showed two demographic peaks, the first around the second half of the XVIII century (1639 inhabitants) and the second at the end of the XIX century (1464 inhabitants). The S/N ratio was low (0.2—0.3) and relatively constant until the beginning of the last century, and only in the last three decades of the XX century, when the population was subjected to immigration, Postua showed a significant increase in the S/N ratio values (to about 0.9). The surname frequency was constant until the 1850 when almost all surnames were the same as those already present in the XVII century. From the first half of the XIX century, the founder surnames decreased, whereas new surnames started to increase and became persistent in the population.
During the last century the structure of the human populations was intensively studied and a great amount of knowledge based on genetic and bio-demographic data was accumulated. Human mating systems provide precious information for population genetics, since inbreeding is a key variable affecting the genetic structure of populations. In this context, the isonymic method has been generally accepted in population genetic studies and surnames have been successfully used as genetic markers in the study of human populations [1, 2]. Because of the large amount of historical data available for European populations, the surname approach represents an efficient tool to describe the evolution of relationships among human populations for which written documents are available. In Italy, as in most European countries, the surnames are inherited through the paternal lineage, simulating a Y-linked inheritance [3, 4]. From the frequency of surnames it is possible to estimate the marital and random isonymy in males and females, the levels of endogamy and similarity between groups [5]. Studies on small and isolated old communities, such as mountain villages, are particularly interesting from a genetic point of view, because these populations are expected to be genetically homogeneous due to many generations of endogamy. In this scenario Postua, a mountain village of only 578 inhabitants [6] located in Piedmont, a region of North-Western Italy, represents
a distinctive example, where consanguinity was intimately related to socio-cultural habits and was traditionally an important component of the marital structure of the population. Indeed, Postua was characterized in the past by partial geographical and cultural isolation, and by a high frequency of marriages among relatives. Like many mountain villages, it was subjected to a emigration flux, mostly towards France but, differently from other migration histories, Postua's migrants and their descendents kept connections with their native village. Until the Second World War, almost all the migrants returned to Postua to chose their wife [7]. Moreover, previous studies based on mitochondrial DNA polymorphisms, Y-chromosome markers (SNPs and microsatellites) and Alu sequences analyses also described Postua as a moderately closed community mostly unaffected by genetic immigration fluxes in the past [8, 9]. Since Postua seems to represent a interesting model of the genetic micro-structure of Alpine Piedmont communities, the aims of the present work were to investigate the microevolutionary pattern of its population by means of demographic methods and to compare such findings with those available for other isolated Italian and Swiss populations.
MATERIALS AND METHODS
Population data
Postua is a small village in the Sessera Valley, province of Vercelli, Piedmont region (North-Western Italy), on the slopes of the Biellese pre-Alpes (Figure). Its name derives from Latin post, meaning the last settlement of the foothills. In this study, we utilised the parish archives of the Postua "Beata Vergine Assunta" Church for the period 1640—1872 and the Postua Municipality archives for the period 1838—1989. A total of 12915 births, 2808 marriages and 11112 death records were analyzed. Data on consanguineous marriages were taken from ecclesiastic dispensations.
Demographic analysis
Natality and mortality rates at different age ranges were calculated by decade. The amount of migration was estimated by the S/N ratio, where S represents the number of surnames and N the number of individuals of the population. In this study we used the surnames present in both certificates of birth and baptism. Typically, isolated populations show a small number of frequently repeated surnames, while large populations are characterized by a high number of rarely repeated surnames. In order to estimate the genetic isolation of Pos-tua from the richness of surnames in the population, we used the Fisher a-parameter [10], a = (N v)/(1 — v), where N represents the population size and v the entry rate of new surnames in the population. As v increases, 1 — v diminishes and the richness of surnames increases. In order to evaluate the population reproductive isolation we calculated the endogamy rate, i.e. the percentage of individuals whose spouse belongs to the same population. Observed marriage isonymy was calculated using the method described in Crow (1980) [11], with I = ES((/N, where SH is the number of couples with the same surname and N the total number of couples. We also used the Lasker's method (1977) [12] to calculate the random isonymy Ia = ES(1Si2/N1N2, where S(1 = is the number of grooms with ith surname, Si2 = is the number of brides with ith surname, N1 = ZSi, and N2 = Si2. The inbreeding coefficient F was estimated using the formula in Crow and Mange (1965) [1], F = Fr (1 - Fn) + Fn, where Fr = (LSl1S!2)/4N1N2 and represents the random component, Fn = (1 - ZSaSi2/N1N2)/4(1 - "LStiSa/NiN2) and represents the non-random component, S(1 = number of grooms carrier the ith surname, Si2 = number of brides carrier the th surname, N1 = total number of grooms, N2 = total number of brides, and I = observed frequency of isonomic couples. Finally, we evaluated the coefficient of consanguinity using the ecclesiastic dispensations. From each dispensation, data were taken on birthplace, given name, paternal and maternal surnames, age at marriage, and degree of the biological relationship up to third cousins. In this case, the inbreeding coefficient was estimated using the Sutter and Tabah (1948) [13] formula, modified by Jacquard (1974) [14]:
Postua
Geographical position of Postua (08°14'E-45°43'N).
C = 1/8 f3 + 1/16 fA + 1/32f + 1/64 f6 + 1/8 fd, where f3 = marriage frequency between uncle-niece or aunt-nephew, f = marriage frequency between first cousins, f5 = marriage frequency between first cousins once removed, f6 = marriage frequency between second cousins, and fd = marriage frequency between second cousins once removed.
RESULTS
The Postua demographic documentation begins in 1640 with a nucleus of 488 inhabitants (Table 1). The population showed two demographic peaks, the first around the second half of the XVIII century (176069 decade, with 1639 inhabitants) and the second at the end of the XIX century (1880-89 decade, with 1464 inhabitants). At the beginning of the last century the natality rate became equal or lower than the mortality rate, thus determining a decrease of the population size to 543 inhabitants in the 1980-89 decade. Mortality rate at different age ranges is reported in Table 2. Up to the beginning of the XX century the infant mortality rate (during the first year of life) was very high in Postua, reaching 44% of the total deaths at the end of the XIX century. With the beginning of the last century this tendency was reversed, probably due to the improved population living conditions and the de-
Table 1. Demographic parameters in Postua population for each decade from 1640 to 1989
Decades N Marriage rate Birth rate Death rate S/N Founders' surnames frequency New surnames frequency a
1640-49 488 0.119 0.069 0.030 0.414 1.000 0.000
50-59 614 0.108 0.051 0.030 0.273 0.889 0.055 35.735
60-69 753 0.092 0.047 0.022 0.341 0.702 0.234 230.041
70-79 894 0.083 0.046 0.025 0.324 0.729 0.292 368.538
80-89 1034 0.106 0.045 0.024 0.241 0.717 0.245 335.536
90-99 1175 0.074 0.031 0.029 0.282 0.735 0.224 339.175
1700-09 1189 0.077 0.039 0.038 0.229 0.881 0.119 160.603
10-19 1198 0.080 0.042 0.035 0.271 0.731 0.192 284.673
20-25 648 0.119 0.046 0.033 0.266 0.521 0.136 203.842
40-49 1478 0.055 0.040 0.035 0.259 0.952 0.095 155.149
50-59 1534 0.078 0.041 0.032 0.250 0.667 0.200 383.5
60-69 1639 0.068 0.033 0.036 0.223 0.840 0.160 312.19
70-79 1582 0.061 0.032 0.031 0.278 0.722 0.14
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.