Dissertação
Dinâmica evolutiva de mandioca (manihot esculenta crantz) em três tipos de solo manejados por caboclos na região do médio Rio Madeira, Amazonas
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Instituto Nacional de Pesquisa da Amazônia - INPA
Resumo
Manioc (Manihot esculenta Crantz) is the most important food crop worldwide that originated
in Amazonia. Bitter manioc varieties are one of the most important food staples for traditional
peoples in Central Amazonia, and a growing body of studies has increased our understanding
of the evolutionary dynamics of the crop under traditional cultivation. However, most of these
studies have been undertaken in single plots or in communities with access to a single soil
type, and, in the case of Amazonia, generally Oxisols and Ultisols in non-flooded upland
plateaus on the terra firme, despite the observations that bitter manioc cultivation is also
practiced in highly fertile soils. Recently, ethnobotanical observations along middle Madeira
River showed that numerous communities of smallholder farmers grow bitter manioc in the
highly fertile soils of the floodplain and Amazonia dark earths (ADE), and in the clayey
nutrient-poor Oxisols. It was observed that, in this region, farmers manage distinct sets of
varieties for each soil type, and those varieties grown in the floodplain and ADE have similar
characteristics. Such observations raised the hypotheses that communities in which farmers
grow manioc in different soils maintain higher genetic diversity than communities in which
manioc is grown on fewer soil types, and that the genetic structure of varieties would be
related to soil types, with special emphasis on the relationship of varieties grown in the
floodplain with those grown in ADE. To test these hypotheses, this study evaluated the
genetic diversity, based on 10 microsatellite markers, of bitter manioc varieties traditionally
cultivated in different soil types (namely ADE, Oxisols and floodplain) along the middle
Madeira River region. Varieties were sampled in two distinct schemes to evaluate the
distribution of genetic diversity on a local scale, as well as intra-varietal genetic diversity. For
the first scheme, it was observed that floodplain varieties had greater genetic diversity (Ā=
5.2; HO= 0.606) than varieties grown on ADE (Ā= 4.5; HO= 0.538) and on Oxisols (Ā= 4.2;
HO= 0.559). Floodplain varieties were also strongly differentiated from the varieties grown on
ADE (FST = 0.108) and Oxisols (FST = 0.093), while these latter two soils were less
differentiated (FST = 0.016). For the second scheme, high intra-varietal genetic diversity was
observed, along with significant differentiation among varieties, with a tendency for varieties
having equivalent names, but grown on ADE and floodplain, to be genetically differentiated.
Additionally, gene flow was detected among some of the varieties. When taken together, the
results of these two sampling schemes reveal that along the middle Madeira River the
traditional farmers maintain high levels of genetic diversity within and among the bitter
manioc varieties grown in different soil types. Higher levels of genetic diversity are not
necessarily found in the communities in which farmers plant bitter manioc on more than one
soil type: higher genetic diversity was observed for communities located in the floodplain.
The hypothesis of closer relationships among varieties according to the soil types in which
they are grown is partly true, since the varieties are genetically structured among different soil
types, but, contrary to expectations, there seems to be an important genetic differentiation
between varieties grown in the floodplain and varieties grown in upland soils (ADE and
Oxisols). In spite of such differentiation, it was demonstrated that some varieties collaborate
to the genetic diversity found within others, irrespective of whether they are from the same
soil type or not. This study adds a new component to the discussion on manioc evolutionary
dynamics, since it is the first time that differentiation of manioc varieties among environments
of cultivation in Amazonia is examined with molecular data.
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