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A populationwide coalescent analysis of Icelandic matrilineal and patrilineal genealogies: evidence for a faster evolutionary rate of mtDNA lineages than Y chromosomes - PubMed

. 2003 Jun;72(6):1370-88.

doi: 10.1086/375453. Epub 2003 Apr 29.

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A populationwide coalescent analysis of Icelandic matrilineal and patrilineal genealogies: evidence for a faster evolutionary rate of mtDNA lineages than Y chromosomes

Agnar Helgason et al. Am J Hum Genet. 2003 Jun.

Abstract

Historical inferences from genetic data increasingly depend on assumptions about the genealogical process that shapes the frequencies of alleles over time. Yet little is known about the structure of human genealogies over long periods of time and how they depart from expectations of standard demographic models, such as that attributed to Wright and Fisher. To obtain such information and to examine the recent evolutionary history of mtDNA and Y-chromosome haplotypes in the Icelandic gene pool, we traced the matrilineal and patrilineal ancestry of all 131,060 Icelanders born after 1972 back to two cohorts of ancestors, one born between 1848 and 1892 and the other between 1798 and 1742. This populationwide coalescent analysis of Icelandic genealogies revealed highly positively skewed distributions of descendants to ancestors, with the vast majority of potential ancestors contributing one or no descendants and a minority of ancestors contributing large numbers of descendants. The expansion and loss of matrilines and patrilines has caused considerable fluctuation in the frequencies of mtDNA and Y-chromosome haplotypes, despite a rapid population expansion in Iceland during the past 300 years. Contrary to a widespread assumption, the rate of evolution caused by this lineage-sorting process was markedly faster in matrilines (mtDNA) than in patrilines (Y chromosomes). The primary cause is a 10% shorter matrilineal generation interval. Variance in the number of offspring produced within each generation was not an important differentiating factor. We observed an intergenerational correlation in offspring number and in the length of generation intervals in the matrilineal and patrilineal genealogies, which was stronger in matrilines and thus contributes to their faster evolutionary rate. These findings may have implications for coalescent date estimates based on mtDNA and Y chromosomes.

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Figures

Figure 1
Figure 1

Coalescent structures of matrilineal and patrilineal genealogies. These were determined by tracing back in time from females and males born after 1972 (grey circles). The black circles represent matrilineal or patrilineal ancestors who have descendants in the contemporary cohort. The number of offspring who leave descendants in the contemporary cohort is shown next to the black circles. As we move back in time, each coalescent event reduces the total number of matrilines or patrilines, until we are left with the single earliest ancestor in the ancestor cohort. After the coalescent tracing procedure has ended, it emerges that this ancestor has eight descendants in the contemporary cohort. White circles represent individuals with no descendants in the contemporary cohort. Such individuals are invisible to the coalescent approach. Coalescent events are not recorded for these individuals, and they are not counted among the offspring attributed to their mother or father. Hence, information is only stored about direct matrilineal and patrilineal ancestors of individuals in the contemporary cohort. Note that generation intervals vary in length, with the result that three descendants from the contemporary cohort are linked to the ancestor through four generations, whereas five such descendants are linked through five generations.

Figure 2
Figure 2

Contributions of the 1848–1892 ancestors to the contemporary cohort. The areas of the circles on the left represent all Icelandic females born between 1848 and 1892 (top) and after 1972 (bottom) who survived early childhood. The black slice in the bottom-left circle represents the proportion of females born after 1972 who could be traced successfully to a matrilineal ancestor born between 1848 and 1892. The black slice in the top-left circle represents these matrilineal ancestors as a proportion of the entire 1848–1892 cohort. The white slice in the bottom-left circle represents the 5,318 contemporary females who could not be traced to matrilineal ancestors born between 1848 and 1892. The white slice in the top-left circle represents the 24,776 females born between 1848 and 1892 who either do not have matrilineal descendants born after 1972 or could not be linked to matrilineal descendants in the genealogical database. The circles on the right present equivalent information for males and patrilines. The parameter stands for the average number of generations between members of the ancestor cohort and their descendants in the contemporary cohort.

Figure 3
Figure 3

Contributions of the 1698–1742 ancestors to the contemporary cohort (see legend for fig. 2)

Figure 4
Figure 4

Histograms of descendants per ancestor for matrilines and patrilines (1848–1892 ancestor cohort)

Figure 5
Figure 5

Histograms of descendants per ancestor for matrilines and patrilines (1698–1742 ancestor cohort)

Figure 6
Figure 6

Changes in the average length of matrilineal and patrilineal generation intervals by birth year of offspring from 1698 to 2000. Generation intervals are defined as the birth year of an offspring subtracted from the birth year of the parent.

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