Post by Admin on Apr 17, 2017 20:01:51 GMT
Fig. 1. The matrilineal descendants of ancestor 1, born 1720, which were sequenced in this study. Each individual in the matrilineal pedigree is identified by a number and circle. White circles represent individuals identified as carriers of the C1 lineage from control region sequences produced prior to this study.
Historical and archeological evidence suggests that
Iceland was first settled by people from Scandinavia and
the British Isles around 870 AD (Smith, 1995). Analyses
of Y chromosome variation indicate that contemporary
Icelanders trace 75–80% of their patrilineal ancestry to
Scandinavia and 20–25% to Scotland and Ireland (Helgason
et al., 2000; Goodacre et al., 2005). In contrast,
studies of mtDNA variation indicate that only 37% of the
population’s matrilineal ancestry can be traced to Scandinavia,
with the remaining 63% deriving from Scotland
and Ireland (Helgason et al., 2001; Goodacre et al., 2005;
Helgason et al., 2009).
Although most of the mtDNA and Y-chromosome lineages
encountered in Icelanders are consistent with an origin in
these neighboring populations, at least one, an mtDNA lineage
that belongs to haplogroup C1, may have a more distant
origin. On the basis of their contemporary geographical distribution
in Native Americans and East Asians, mtDNA
sequences from haplogroup C1 are thought to originate from
groups that participated in the settlement of the Americas
around 14,000 years ago (Starikovskaya et al., 2005; Tamm
et al., 2007). In the first study of Icelandic mtDNA variation,
two identical copies of control region sequences belonging to
the C1 lineage were encountered in a sample of 401 individuals,
which differed from the Cambridge Reference Sequence
at 11 positions (73G 152C 249d 263G 290-291d 16223T
16298C 16311C 16325 16327T) (Helgason et al., 2000). In
that study, this lineage was assumed to be either a recent
(20th century) arrival from the Americas or East Asia or
traceable to an ancient female settler from Scandinavia with
Asian matrilineal ancestry (Helgason et al., 2000).
Fig. 2. The matrilineal descendants of ancestor 2. The
matrilineal descendants of ancestor 2, born 1740.
Because of the low frequency (0.26%) of the C1 lineage
in the existing sample of 1538 Icelandic mtDNA sequences,
the deCODE genetics genealogical database was
used to help minimize the amount of additional sequencing
needed to identify new carriers and matrilineal
ancestors. We first traced the matrilines of the 1538
sequenced individuals back to their earliest known
ancestors. This yielded 792 distinct ancestors, from
whom matrilines were then traced forward in time to a
total of 250,242 of 408,915 Icelanders born between 1895
and 2005. On the basis of the very low error rate (\1%)
of matrilineal genealogies (Sigurdardottir et al., 2000), it
was assumed that the mtDNA sequence of these individuals
is identical to that of their sequenced matrilineal
relative. We therefore sought to preferentially sample
potential new carriers of the C1 lineage from the
remaining 158,673 Icelanders born 1895 to 2005, whose
mtDNA sequence was defined as unknown.
The mtDNA control region (positions 16055–16410) was sequenced
for 353 Icelanders, who were descendants of 304 previously
unsampled ancestors. These turned out to include
two new carriers of the C1 lineage, traceable to two distinct
matrilineal ancestors, referred to hereafter as
ancestors 3 (born 1710) and 4 (born 1720) (see white
circles Figs. 3 and 4, respectively). The C1 carrier status
of all four ancestors, 1, 2, 3, and 4, was confirmed by
sequencing the mtDNA control region (positions 15811–
775) in 76 additional matrilineal descendants (25 for
ancestor 1, 14 for ancestor 2, 24 for ancestor 3, and 13
for ancestor 4, see gray circles Figs. 1–4). As the matrilineal
descendants of all four ancestors carry identical
mtDNA control region sequences, it is likely that their
most recent common ancestor (MRCA) through the
female line was born after the settlement of Iceland in
870, but no later than 1700.