voici ce que j'ai trouvé sur un site qui pour moi résume bien la situation et va dans le sens du dernier message de MREG (malheureusement c'est de l'anglais pour ceux qui ont du mal), en gros ça dit que la composante arabe des malgaches est principalement Swahilis de mélange Omanite (arabe) + africain (bantou) et qu'ils se sont mariés aux femmes locales en arrivant à Madagascar. Il est a noté que le vocabulaire concernant les femmes à madagascar est d'origine indonesienne alors que pour les hommes, l'influence des langues bantoues et Swahilis est visible, montrant que ce sont biens des hommes "arabes" ou metisses arabes africains qui sont arrivés de la mer et ont pris des femmes locales. Il parle aussi de marchands perses et indiens. Les vrais arabes sont aussi passés par la, mais sans grande influence genetique.
Ensuite il parle d'une exception etant les Antaimoro, qui d'apres les recherches seraient descendant des somaliens de la region Ogaden d' Ethiopie.
"V. The Arabs
Some linguistic and cultural influence was exerted by the so-called Arabs, mainly visitors from the Omani colonies along the East African coast. The Omanis, colonized by the Portuguese in the 16th century, had quickly learned to copy Portuguese shipbuilding technology. Later they freed themselves of their masters and then, step by step, evicted them from their colonies along the African coast, thus establishing their own Omani empire which lasted until the 1960s when Pakistan occupied the last Omani colonies on the subcontinent, Gwadar and Pasni, and when Zanzibar became part of Tanzania.
By driving out the Portuguese and mixing with the local population, the Omanis created the colonies of Lamu, Malindi, Mombasa, Zanzibar and Pemba, and Sofala, and established themselves in the Comoro Islands. The "Arabs", practically Muslim Swahilis of mixed Omani-African origin, regularly visited Madagascar, traded and mixed with the local women, without, as a rule, attempting to establish any formal rule or proselitizing the locals. Generally, no mosque or medresse was built but the locals learned to shun pork meat and dogs, and to circumcise their sons. The "Arabs" usually came (in the Manajary region) to attend their sons' circumcision feast. Arab and Swahili numbers and words were adopted by the Malagasy language, and the Antaimoro dialect was written in Arabic script long before the Merina dialect.
Since other Arab merchants, principally Yemenis and Kuwaitis, were also traveling along the East African coast buying timber, ivory and slaves, it is likely that they -- as well as traders from India and Sri Lanka (Ceylon)-- regularly visited Madagascar. On the whole, however, it is safe to assume that the cultural influence of the "Arabs" vastly exceeded their contribution to the Malagasy gene pool. Since they mainly spoke Swahili, a mixed Bantu-Arab language (Arabic for "Language of the coast"), it can be assumed that they were the principal originators of the Bantu words in Malagasy. The Swahilis, apart from being seafarers, were mainly pastoral people (farming only came in the 19th century to large parts of East Africa), and introduced the Bantu terms in Malagasy related to cattle ranching which they promoted among the Malagasy tribes (including the Merina).
There were, however, some exceptions of this general pattern of Arab influence in Madagascar. One example is the Amtaimoro tribe in southeastern Madagascar. Research has shown that the ancestors of the small Antaimoro elite of royals and priests were probably not Arabs but Somalis from the Ogaden region of Ethiopia.
In the 17th century there were a number of Swahili colonies on the West and Northwest Coast. Some of the colonies were founded and governed by Swahilis from Malindi and Mombasa and served as nodes of the slave trade. These colonies had mosques, medresse and all paraphernalia of Malindi culture.
At least one more colony had been established by runaway slaves of the Swahili who found freedom on Madagascar. These slaves had escaped from Djibouti, Mogadishu, and all Swahili places southward to Sofala.
Generally, these Arabs, Swahilis and Somalis were called "cafres" in the 17th century European literature. However, their numbers remained small and their colonies either dissolved or were overran in the 18th century by the Sakalava tribe which, coming from the South, moved up the West Coast and established its surprisingly modern governance system over most of the island.
The precise origins of the Malagasy will at some future date become fully known as a result of the EU-funded Human Genome Diversity Project directed by the eminent Italo-American geneticists Luigi Luca & Francesco Cavalli-Sforza."