柳葉菜科(Onagraceae)植物泛分布於世界各地，約含有7族17屬652種，其中水丁香屬(Ludwigia)植物泛分布於全球熱帶至溫帶地區的溼地，含有23組81種，其中小果組(section Microcarpium)含有14種，除 L. stricta特產於古巴外，其餘種類主要分佈於美國東南部，所有種類皆生長於積水地、淺水池、湖、河、溪、瀉湖、沼澤、潮濕的牧場或草原、排水或灌溉溝渠與森林內的沼澤。本研究取全部14種小果組水丁香以葉綠體DNA atpB-rbcL 基因間的非轉譯區間序列與細胞核的核糖體DNA (nrDNA)內轉錄區間(Internal Transcribed Spacer, ITS)的序列，進行小果組植物親緣關係之分析。由ITS分析結果除L. stricta因所得結果無法分析外，針對其餘13個種類以ITS序列進行親緣關係，分析結果可分成兩個主要類群，第一個主要類群由方果類群(L. alata, L. sufruticosa, L. lanceolata)及一新種L. pseudoalata，與毛果水丁香複合群(L. pilosa complex, L. pilosa 與 L. ravenii)組成；毛果水丁香(L. pilosa)與雷氏水丁香(L. ravenii)由分析資料顯示確為已分化分類群，但彼此親緣關係接近。第二個主要類群則由其他種類組成，可得到以下幾個結果，(1)是L. polycarpa、L. glandulosa類群與有瓣類群(L. linearis, L. linifolia)為分化明顯的類群，其中L. glandulosa類群包含有L. glandulosa及其亞種 L. glandulosa ssp. brachycarpa，分析顯示這兩個分類群應已分化；(2)為圓果水丁香(L. sphaerocarpa)與該種類與其他種類的複雜情形，顯示該物種應為雜交起源，並與其他物種尚有複雜的基因交流；(3)為小葉類群(L. curtissii, L. simpsonii, L. microcarpa)，結果顯示小果水丁香(L. microcarpa)與另兩種分化明顯，柯氏水丁香Ludwigia curtissii與辛氏水丁香(L. simpsonii)可能為同一種類。

Onagraceae are widely distributed in the world, which include about 7 tribes, 17 genera, and 652 species. Genus Ludwigia are widely distributed in wetlands of tropical and temperate regions in the world, which include 23 sections and 81 species. The section Microcarpium has 14 species, all are mainly in ES of U.S.A. excepting that L. stricta is endemic to Cuba. All species of Ludwigia section Microcarpium grow at wetlands, including habitats of shallow ponds, lakes, creeks, lagoons, marshes, humid ranches grasslands, drains, agricultural ditches, and wetlands in forest.
This research uses all of the 14 species in Ludwigia section Microcarpium, doing the phylogenetic relationship analysis by using the chloroplast DNA atpB-rbcL, the sequence of noncoding spacer, and the sequence of Internal Transcribed Spacer (ITS) of the ribose DNA (nrDNA) of cell nucleus. The result of ITS analysis shows that, excepting that L. stricta cannot be analyzed, the phylogenetic relationship of the rest of the 13 species can be divided into two main groups. The first one consists of tetracarpa taxon (L. alata, L. sufruticosa, and L. lanceolata), a new species L. pseudoalata, and L. pilosa complex (L. pilosa, and L. ravenii). Analysis data shows that L. pilosa and L. ravenii are two differentiated taxa, but with close parental relationship. The second one consists of the rest of species, and the following results concluded: (1) L. polycarpa, L. glandulosa taxon, and petalous group (L. linearis, L. linifolia) are significantly differentiated. Analysis also shows that L. glandulosa, and its subspecies L. glandulosa ssp. brachycarpa should have already differentiated. (2) The complicate relationship between L. sphaerocarpa and other species shows that this species should be the origin of hybridization, and has complicate gene flow with other species. (3) The L. microcarpa in microphyllum group is significantly differentiated from the other two species of the same group -- Ludwigia curtissii and L. simpsoni, which might be the same species。

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