MOLECULAR PHYLOGENY AND PHYLOGEOGRAPHY
A molecular phylogenetic analysis uses evolutionary changes at the molecular level (usually in DNA sequences) to reconstruct the evolutionary relationships within a group of organisms. The figure below is a molecular phylogeny (also, “cladogram” or “tree”) showing hypothetical relationships among species of the bryozoan genus Cauloramphus, which has high species diversity in the North Pacific.
Neighbor-joining tree of Cauloramphus species based on analysis of a 658 bp fragment of the mitochondrial gene cytochrome oxidase c subunit 1 (COI) using Log-Det distances, with third codon positions omitted.
Two-letter abbreviations following some names indicate the locality (AS, Akkeshi, Hokkaido, Japan; KE, Ketchikan, Alaska, USA; KO, Kodiak, Alaska, USA; OS, Oshoro, Hokkaido, Japan; WA, Anacortes, Washington, USA).
Numbers near nodes indicate bootstrap values in percent determined from analyses of the entire data set by maximum parsimony (MP), maximum likelihood (ML), and neighbor-joining (NJ), and posterior probability values X 100 from a Bayesian analysis of the entire data set.
Phylogeography is the investigation of the principles and processes governing the geographic distributions of evolutionary lineages of organisms, especially those within and among closely related species (Avise, 2000).
Although the backbone nodes (branching points) in the tree above are not well supported, other, well-supported nodes permitted an assessment of intra- and interspecific divergence (in terms of genetic distance) in Cauloramphus, and allowed us to examine how genetic distance correlated with morphological divergence in characters used in Cauloramphus taxonomy.
Kimura 2-parameter (K2P) distances within local populations of five morphospecies (in bryozoans, species defined on the basis of characters of skeletal morphology alone) ranged from 0.16 to 3.01%. For three morphospecies, K2P distances ranged from 0.50 to 11.0% between populations separated geographically by 750–4500 km, with no correlation between genetic distance and geographical separation.
Our results indicated that the lack of detectable morphological differences between geographically separate populations of a morphospecies says little about the genetic distance between the populations.
The following two figures illustrate the lack of correlation among genetic distance, morphological divergence, and geographical separation.
The K2P distance (COI ) for populations of C. spinifer between Akkeshi, Japan and Kodiak, Alaska is 2.3%; the populations are morphologically indistinguishable; and the geographical separation is 4500 km.
Comparison of morphology between DNA voucher specimens of Cauloramphus spinifer from two localities: (A, B) Kodiak, Alaska, USA; (C, D) Akkeshi, Hokkaido, Japan. (A, C) Dried, unbleached autoozoids. (B, D) Bleached autozooids. Scale bars, 0.50 mm.
In contrast, the K2P distance (COI ) for populations of C. multispinosus between Akkeshi, Japan (Pacific coast of Hokkaido) and Oshoro, Japan (Sea of Japan coast of Hokkaido) is 11.0%; the populations are morphologically indistinguishable; and the geographical separation is only 750 km.
Comparison of morphology between DNA voucher specimens of Cauloramphus multispinosus from two localities: left column (A, C, E), Oshoro, Hokkaido, Japan; right column (B, D, F) Akkeshi, Hokkaido, Japan. (A, B) Dried, unbleached autozooids. (C, D) Bleached autozooids. (E, F) Kenozooidal ooecia in bleached zooids. Scale bars, 0.50 mm.
Our results also indicated that non-overlapping differences in the ranges of one or more characters indicate substantial genetic divergence and probable reproductive isolation. For example, the only morphological difference we detected between C. cheliferoides in the western Aleutian Islands, Alaska, and C. oshurkovi on the Pacific side of Kamchatka was a non-overlapping difference in the number of opesial spines (compare A and D below), yet the genetic distance between the two was 19.0%.
(A–C) Cauloramphus cheliferoides. (A) Autozooids, unbleached. (B) Autozooids, bleached. (C) Distal end of bleached zooid showing raised kenozooidal ooecium (asterisk). (D–F) Cauloramphus oshurkovi. (D) Autozooids, unbleached. (E) Autozooids, bleached. (F) Distal end of bleached zooid showing kenozooidal ooecium (asterisk). Scale bars: 0.5 mm (A, D), 0.25 mm (B, E), 0.05 mm (C, F).
Dick MH, Mawatari SF, Sanner J, Grischenko AV (2010) Cribrimorph and other Cauloramphus species (Bryozoa: Cheilostomata) from the northwestern Pacific. Zoological Science: in press.
Dick MH, Hirose M, Mawatari SF. Molecular distance and morphological divergence in Cauloramphus (Cheilostomata: Calloporidae). Proceedings of the 15th International Bryozoology Association Conference, 1-8 August 2010, Kiel Germany. In review