Tiny segments from the same common ancestors

Disclaimer: This post demonstrates the use of 1+cM when comparing specific groups of people in order to see patterns of multiple descent from a few ancestors.  It should not be used to validate connections with matches, particularly in this example where connections are beyond a genealogical time frame reaching at least up to 500 years.

Recently I have been comparing both western Polynesian (Tongan and Samoan) and eastern Polynesian (Hawaiian and Maori) matches.  I compared western Polynesians among themselves, and  did the same thing with eastern Polynesians comparing them among themselves.  Then I compared the two groups to each other.

To those who are not familiar with Polynesian origins and/or are new to reading my blog, I will recap.  The ancestors of Polynesians originated from the Melanesia area and thrived there for thousands of years. Thousands of years later a group of “Austronesians” originating from Southeast Asia moved into the area, intermingled briefly and continued to move into western Polynesia where Polynesian culture was born.  At least a couple of thousand of years would pass before they would continue to expand further eastward.  As Polynesians moved from west to east, their genome became less diverse due to repeated founder’s effects and bottle necking.

oceania

I analyzed my mother’s results and compared her to a Hawaiian (orange), and a Maori (blue) below.  The Hawaiian is her top match, sharing a total of 693.60cM, longest block 15.52cM, consisting of 158 segments.  The Maori is her 4th top match sharing a total of 517.90cM, longest block 18.08cM, consisting of 119 segments.  FTDNA counts all the tiny segments as low as 1cM once the criteria of a match is met, which is why the number of segments is high.

tinyseg-mom

With the default at 5+cM I did not see anything unusual other than ordinary small segment matches.  But when I reduced the setting down to 1+cM (above), you can see a lot of tiny segments resembling a comb.  The slightly bigger gaps are just the missing teeth of a comb.  Some of these patterns begin to appear at 3+cM, although most do not appear until you reduce it down to 1+cM.  In my mother’s example above I show only chromosomes 1 – 20 since there were no segments that looked like a comb on the other chromosomes.

Then I looked at a Maori woman’s results (below) and compared hers to other Maoris and one Hawaiian.  She also shows the missing teeth at 1+cM, but only in a few areas.  Some areas have the comb pattern while other areas seem random.  The random segments could be IBS (Identical by State) or IBD (Identical by Descent).  Polynesians lack genetic diversity, particularly eastern Polynesians more than western Polynesians, so the random looking segments could be both IBS and IBD segments.

tinyseg-mary

Then I looked at two Tongan men and compared them to other Tongans and Samoans.  With Tongans & Samoans there seem to be more randomness.  A few of the tiniest segments may be close to each other, but nothing resembling too much like my mother’s results, a definite comb-pattern.  Take the purple and green colors for example for this one Tongan man below.  Notice how on some chromosomes they seem to be closer together while on others it just looks random.  Again, these are only using the bare minimum 1+cM.

tinyseg-peni

The other Tongan example.

tinyseg-keni

As you can see, it is hard to look for patterns that resembles a comb, and instead you see random colors all over the chromosomes.  What was interesting to see was how little X these Tongans had.  Unlike with the Maoris and Hawaiians, many of them shared multiple segments with each other.

But what does all of this mean?  These are very small island populations.  They have had repeated emigration from these small islands that resulted in a series of founder’s population.  There there was also bottle necking that occurred a few times.  All of these combined would leave only a few closely related ancestors to populate and repopulate new areas every time.

So the multiple, very small segments that represents a comb with missing teeth is the result of people descending from just a few ancestors who contributed that particular segment, but was inherited from multiple lines going back to the same ancestor over and over again.

Below is an image where I compare my mother with two Samoans (yellow & green) and three Tongans (orange, blue & purple).  There seems to be more randomness, however, there are a few of those comb patterns.

tiny-mom&western

Notice how the X chromosome is much more full, unlike what we saw when comparing the western Polynesians (Tongans & Samoans) among themselves. The yellow color belongs to a Samoan woman. The fact that women have 2 X chromosomes may be the reason why there is a long match versus using two Tongan men whose matches included two women in their examples above.  But these are Polynesians, so you would expect more of a match on the X.  My observance of matches for the past 2 years was limited to only my mother being compared to others, which means I have seen a lot of X matches for her, and the same for myself and my brother.

From what I am noticing so far is that these patterns look like what is mentioned in research papers about Polynesian genome and the loss of heterozygosity going from west to east.  The last place in Polynesia to be settled was in the east, ending at the extreme points of the Polynesian triangle, namely Rapa Nui (Easter Island) in the south east, Aotearoa (New Zealand) to the south west, and the Hawaiian islands in the north.  This explains why my mother and the Maori woman have less random looking tiny segments compared to the Tongans and Samoans.  And if we compare western and eastern Polynesians to each other, we may see some randomness but not as much as we would see with western Polynesians alone.  Other types of Polynesians getting DNA tested would help to exhibit any other additional patterns that I cannot currently see with the majority of Hawaiians and Maoris getting tested.

Loss of heterozygosity – from Western Polynesia to Eastern Polynesia

Genetic research on Polynesians will frequently mention the loss of heterozygosity.  This is more noticeable when comparing eastern Polynesians to western Polynesians.

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Map outlining migratory paths of Austronesian speaking populations, including estimated dates. Adapted from Bellwood et al., (2011) “Are ‘Cultures’ Inherited? Multidisciplinary Perspectives on the Origins and Migrations of Austronesian-Speaking Peoples Prior to 1000 BC.” [doi: 10.137/journal.pone.0035026.g001

Polynesian populations are relatively homogenous both phenotypically and genetically. Over a span of 3,200 years they moved throughout the Pacific, and unlike in Europe and other large continents, they did not mix with other populations due to isolation.  These small founder populations have experienced several bottleneck effects, which further caused this loss of heterozygosity ending with the settlement of eastern Polynesia.  Polynesians’ lack of genetic diversity is less evident in western  Polynesia where initial settlement began.  Hawai’i, New Zealand and Easter Island are considered to be eastern Polynesia, and these places were the last places of Polynesia to be settled.

Recently I have been able to look at the autosomal matches among Samoans and Tongans of western Polynesia.  Previously, I have been only studying Hawaiian matches and noticed that top matches were both Hawaiians and Maori people.  Looking at Samoans and Tongans was very interesting as I now could compare the two different regions.

My mother is 80% Hawaiian, while I am 40%.  And as admixed as I am, I still get 1st – 3rd cousin predictions on Family Tree DNA (FTDNA), while on 23andme I get 2nd cousin and 3rd to distant cousin predictions.  The centimorgan totals that I show with my matches reach as high as 369cM on FTDNA, and 161cM on 23andme.  For my mother, 693cM on FTDNA and 376cM on 23andme.  I see the same happening with Maoris, ranging between 300cM – 700cM (FTDNA) for the top 20 people.  And for a non-admixed Hawaiian, their top matches are in the 600 – 700cM range.   An admixed Polynesian would logically have lower totals. But even an admixed person can still have a fairly high amount of totals shared, as when I am comparing myself being less than half Hawaiian.

When comparing two Tongans, the highest that they shared was 335cM.   A Samoan compared to another Samoan was 366cM.  And both of these Tongans and Samoans had their remaining top matches in the range of 100cM to 200cM.  Many of their matches are the same Hawaiians and Maori that match each other at a much higher total.  It is amazing to see these autosomal matches and how diverse the western Polynesians are, or rather how Hawaiians and Maoris are not as diverse.  And even if it is an admixed Hawaiian or Maori, the matches to each other are still pretty high, and as high as what non-admixed western Polynesians would have to each other.

When comparing the longest block (largest segment) with Tongans and Samoans, they seem to rarely get close to 15cM, averaging around 10cM.  Anything more than that could indicate a possible closer relationship or perhaps a specific common geographic origin.  The Hawaiians and Maoris usually range between 10cM – 15cM for the largest segment, but can go as high as 28cM which is usually in admixed Hawaiians and Maoris compared to each other.  In other words, all Polynesians in general will have high totals exceeding 100cM, but whose largest segment rarely exceeding 10cM.

I look forward to more western Polynesians getting tested so we can see if there is any pattern to specific islands in their own island group, something I have been trying to do with Hawaiians with the few haplogroups that there are for Polynesians.  What also needs to be analyzed are people from Tahiti and the Marquesas being that they were key dispersal points for eastern Polynesians.  I managed to only see the results of one admixed Tahitian woman and her match totals are identical to mine when comparing totals.  I am curious to find out what non-admixed Tahitians will show, if it is more identical to eastern Polynesians, or to western Polynesians.