Phylogenetic relationships of crown conchs
In this paper the phylogeographical distribution of a marine gastropod snail, the crown conch Melongena is investigated. The crown conch Melongena is a large marine mollusk that belongs to the family Melongenidae, traditionally together with the busycon whelks. This gastropod snail has a variety of different shapes but is commonly characterized by direct-developing, lecithrotropic larvae. The Melongena is endemic to tropical Americas with some exceptions for the Pacific, since it dominantly lives in intertidial zones. In the present paper its phylogeny and geographical disparity is investigated with a representative specimen genomic analysis and the construction of computer-aided phylogenetic trees. Specimens that were analyzed in the present paper were collected from Intertidal Florida and eastern Alabama, USA, Mexico and Panama, and compared to several members of the same subfamily from Taiwan and Singapore, as well as several outgroups, one specimen from the subfamily Busyconiae (whelk) and one from Fasciolaria hunteria.
On the topic of the geographical distribution of specific marine taxa such as Melongena, previous studies have dictated that dispersal ability is a function dependent of larvae duration. Moreover, gene studies have documented that there exist no barriers in the possibility of gene flow from one marine environment to the other, where taxa can disperse in their planktonic forms. The observation however, that several species are distributed in areas far more distanced than the potential distance covered in planktonic form, or that non-planktonic taxa are more dispersed than neighbourly planktonic ones, has raised confusion to the viewing of geographical dispersal in marine ecosystems.
In previous studies, a lot of systematic confusion has accumulated on gastropods such as Melongena, due to numerous and differentiating morphology descriptions at the shell level. Even more, genetics and other-than-shell morphology studies have resulted in rearrangements at the familial taxonomy and even in the description of novel, cryptic species. This background has led the authors to the study to formulate and test three different hypotheses concerning the taxonomy and dispersal of the Melongena spp, and particularly five taxa belonging in the Melongenidae family, referred to hereafter as the corona complex.
The three main purposes of the study were
a) To evaluate interspecies relationships and validate the current taxonomy of the corona complex, based on shell characteristics, shedding light to the ongoing confusion and debate
b) To evaluate the geographical distribution of these species according to their phylogeny (i.e. establish genetic similarity to other symbiotic marine taxa) ,and
c) To validate previous research outcomes that have just placed (2004-5) a previous subfamily of the Busyconinae, in a different family whatsoever (also known as the paraphyletic hypothesis).
The authors in their methodology collected 48 adult specimens ,representatives of all currently recognized species in the genus Melongena from Intertidal Florida and eastern Alabama, USA, Mexico and Panama and 5 more out-group specimens (see above) from Taiwan and Singapore. All snails were anesthetized and deep-frozen and genomic DNA was collected with currently approved in-house or commercial techniques for such a collection (a modified non-boiling chelex-based protocol, an NaCl extraction protocol and the Promega Wizard Genomic DNA Purification Kit (Madison, WI, USA) for samples that were recalcitrant to other methods. A region of the cytochrome c oxidase subunit I (mitochondrial DNA) or COI, and ribosomal 16S DNA were all amplified and sequenced from the specimens, separately in two different experiments. A computer program using maximum likelihood analysis under a model of evolution (PAUP) constructed phylogenetic trees for the specimens for both COI and 16S r-DNA of the samples in question.
The results of this study suggest that
a) In the authors own words our data do not indicate population subdivision, and the low sequence divergence levels observed, indicate that the corona complex is probably a single species. This means that the results of the genomic analyses in the species level (COI and 16S DNA) were extremely low in sequence variability, showing that the possibility that the specimens tested would belong in a different species is also extremely low.
b) That the Busyconinae are rightfully categorized in another subfamily than the Melongena., since they are phylogenetically less similar. In particular, the construction of the phylogenetic trees, with the aid of the computer program, created a much higher likelihood that these two species belong in a different family in the process of evolutionary change, rather than in the same, as had been traditionally thought until 2004.
It is argued in great detail that larval dispersal can play an important role in population structure in marine mollusks. From the background material provided for this study, a lot of confusion has made it hard to understand the mechanisms of geographical preference. The authors suggest that in light of their findings of genetic simplicity rather than differentiation between the corona complex snails, researchers of marine geodiversity should focus on life-history traits, ecological factors or even paleogeographical history when researching the characteristics of phylogeography for the marine species.
Authors are quite informative on the levels of divergence found in Melongena spp. andor related genera of snails, the latter being approximately five to 20 times more divergent than species in the corona complex. The present study supports the designation of only four extant species within the genus M. corona, M. patula, M. melongena and M. bispinosa. Data suggests that M. bispinosa and M. corona are the two most divergent species within the genus, and the authors state that M.bispinosa should no longer be called M.corona bispinosa. Moreover, the high diversity found in the COI analysis of species other than the corona complex, suggests that the corona complex are lower taxonomically (thus older evolution wise) than the other Melongena spp.
This was reflected by the nuclear DNA as well, where both nuclear histone H3 and 28S ribosomal DNA sequences were found to be monomorphic for the corona complex species alone.
An important finding for this particular paper was also the confirmation with genomic phylogenetic analysis that Busyconidae are a distinct family than the Melongena, a finding that adds greatly to the research on marine species phylogeny. In detail, Busyconinae is placed as the sister group to Buccinidae with high confidence, in respect to the analysis of 16S DNA sequences and COI.
Interpreting the above study, one might say that this is a very well written paper with a particular interest in phylogenomics and taxonomy of the species. However this is not the only importance one sees when seeing the article in a larger context. When a species, that has a limited biogeographical disparity ,as documented by biogeographical analysis historically, is not genetically variable nor does it have a large amount of speciation between its members, this genetic simplicity is a finding that is not only proof of the given ecological confinement but also proof of a slower evolution. This paper also clearly adds a controversy to research oriented at larva duration-dependent disparity analysis. This is also because the corona complex organisms are characterized by extremely low vagility per se.
An explanation for the extreme genetic simplicity would be that the corona complex suffered a localized extirpation followed by recolonization some time after the mid- to Late-Pleistocene. Research on this topic should become wider to include other factors that play a major role in this geographical preference, such as more traditional aspects of ecology, adaptation and migration.
The findings of the present paper were analyzed in a simple manner throughout the manuscript with illustrative graphics and in detail concerning the methodology used. A second critical analysis of the science used in this experiment would argue that given the particular task, the authors had to carry a more precise genetic analysis. That is because 16S DNA and COI, are somewhat wide targets to the species level, and are not sufficiently discriminatory (i.e. they are frequently highly preserved) to observe diversity and or genetic polymorphism a micro satellite loci sequencing for example, would offer certainty and speciation to the taxonomical findings (subtler intraspecific genetic variance) and would allow for analysis in population subdivision between areas of habitat. And controversially, the authors do not provide a paragraph that would constitute their choice of this particular genetic analysis as sufficient i.e. stating the methods of analyses characteristics and specificity, nor do they reference previous literature that might have rendered this type of analysis as the gold standard. Moreover, they use phylogeny limits that run back to 1985. To add to these interpretations, the authors admit in their discussion section that the existence of a universal highly conserved COI gene may explain the lack of divergence in corona complex taxa, as has been found for other species.
In total however, the authors are aware and sincere about their limitations and have outlined their contribution to the topic thoroughly and with a sufficient analysis of all pertinent research in the subject.
RELEVANT CHAPTERS
Evolutionary trees the authors try to elucidate the complexity or absence of it, in a marine species, using highly conserved genetic elements, and place one subset of the studied population in a lower evolutionary level than the others. Moreover, they provide evidence of a taxonomical controversy, that allows for a reclassification of whelks in a different family than Melongena.
Migration, drift and nonrandom mating the question of marine migration and the planktonic-form drift constitutes a major argument for this article, where the biogeography of an organism is studied for its selective environmental disparity. Drift is being questioned and no data is found to substantiate the mechanisms of geodisparity in this organism.
Evolutionary analysis of form and function genetic analysis proves low divergence between a selected species that is traditionally confused in its taxonomy due to variability in shell forms.
Phylogenomics and adaptation Phylogenomic methods are being used in this paper to elucidate the place of several subspecies of the Melongena spp. to different subgroups.
Mutation and genetic variation The methods employed by this study do not allow for the identification of mutation, and genetic variation is found to be low within a species.
Evolution at multiple loci This is also a limitation of this study that is not designed to provide with such information for the organism selected.
Life history characters Although geographical disparity is investigated and mapped, there is no explanation to the theory of this organisms particular selection of habitat, although suggestions regarding life history events are hypothesized to play a major role.
On the topic of the geographical distribution of specific marine taxa such as Melongena, previous studies have dictated that dispersal ability is a function dependent of larvae duration. Moreover, gene studies have documented that there exist no barriers in the possibility of gene flow from one marine environment to the other, where taxa can disperse in their planktonic forms. The observation however, that several species are distributed in areas far more distanced than the potential distance covered in planktonic form, or that non-planktonic taxa are more dispersed than neighbourly planktonic ones, has raised confusion to the viewing of geographical dispersal in marine ecosystems.
In previous studies, a lot of systematic confusion has accumulated on gastropods such as Melongena, due to numerous and differentiating morphology descriptions at the shell level. Even more, genetics and other-than-shell morphology studies have resulted in rearrangements at the familial taxonomy and even in the description of novel, cryptic species. This background has led the authors to the study to formulate and test three different hypotheses concerning the taxonomy and dispersal of the Melongena spp, and particularly five taxa belonging in the Melongenidae family, referred to hereafter as the corona complex.
The three main purposes of the study were
a) To evaluate interspecies relationships and validate the current taxonomy of the corona complex, based on shell characteristics, shedding light to the ongoing confusion and debate
b) To evaluate the geographical distribution of these species according to their phylogeny (i.e. establish genetic similarity to other symbiotic marine taxa) ,and
c) To validate previous research outcomes that have just placed (2004-5) a previous subfamily of the Busyconinae, in a different family whatsoever (also known as the paraphyletic hypothesis).
The authors in their methodology collected 48 adult specimens ,representatives of all currently recognized species in the genus Melongena from Intertidal Florida and eastern Alabama, USA, Mexico and Panama and 5 more out-group specimens (see above) from Taiwan and Singapore. All snails were anesthetized and deep-frozen and genomic DNA was collected with currently approved in-house or commercial techniques for such a collection (a modified non-boiling chelex-based protocol, an NaCl extraction protocol and the Promega Wizard Genomic DNA Purification Kit (Madison, WI, USA) for samples that were recalcitrant to other methods. A region of the cytochrome c oxidase subunit I (mitochondrial DNA) or COI, and ribosomal 16S DNA were all amplified and sequenced from the specimens, separately in two different experiments. A computer program using maximum likelihood analysis under a model of evolution (PAUP) constructed phylogenetic trees for the specimens for both COI and 16S r-DNA of the samples in question.
The results of this study suggest that
a) In the authors own words our data do not indicate population subdivision, and the low sequence divergence levels observed, indicate that the corona complex is probably a single species. This means that the results of the genomic analyses in the species level (COI and 16S DNA) were extremely low in sequence variability, showing that the possibility that the specimens tested would belong in a different species is also extremely low.
b) That the Busyconinae are rightfully categorized in another subfamily than the Melongena., since they are phylogenetically less similar. In particular, the construction of the phylogenetic trees, with the aid of the computer program, created a much higher likelihood that these two species belong in a different family in the process of evolutionary change, rather than in the same, as had been traditionally thought until 2004.
It is argued in great detail that larval dispersal can play an important role in population structure in marine mollusks. From the background material provided for this study, a lot of confusion has made it hard to understand the mechanisms of geographical preference. The authors suggest that in light of their findings of genetic simplicity rather than differentiation between the corona complex snails, researchers of marine geodiversity should focus on life-history traits, ecological factors or even paleogeographical history when researching the characteristics of phylogeography for the marine species.
Authors are quite informative on the levels of divergence found in Melongena spp. andor related genera of snails, the latter being approximately five to 20 times more divergent than species in the corona complex. The present study supports the designation of only four extant species within the genus M. corona, M. patula, M. melongena and M. bispinosa. Data suggests that M. bispinosa and M. corona are the two most divergent species within the genus, and the authors state that M.bispinosa should no longer be called M.corona bispinosa. Moreover, the high diversity found in the COI analysis of species other than the corona complex, suggests that the corona complex are lower taxonomically (thus older evolution wise) than the other Melongena spp.
This was reflected by the nuclear DNA as well, where both nuclear histone H3 and 28S ribosomal DNA sequences were found to be monomorphic for the corona complex species alone.
An important finding for this particular paper was also the confirmation with genomic phylogenetic analysis that Busyconidae are a distinct family than the Melongena, a finding that adds greatly to the research on marine species phylogeny. In detail, Busyconinae is placed as the sister group to Buccinidae with high confidence, in respect to the analysis of 16S DNA sequences and COI.
Interpreting the above study, one might say that this is a very well written paper with a particular interest in phylogenomics and taxonomy of the species. However this is not the only importance one sees when seeing the article in a larger context. When a species, that has a limited biogeographical disparity ,as documented by biogeographical analysis historically, is not genetically variable nor does it have a large amount of speciation between its members, this genetic simplicity is a finding that is not only proof of the given ecological confinement but also proof of a slower evolution. This paper also clearly adds a controversy to research oriented at larva duration-dependent disparity analysis. This is also because the corona complex organisms are characterized by extremely low vagility per se.
An explanation for the extreme genetic simplicity would be that the corona complex suffered a localized extirpation followed by recolonization some time after the mid- to Late-Pleistocene. Research on this topic should become wider to include other factors that play a major role in this geographical preference, such as more traditional aspects of ecology, adaptation and migration.
The findings of the present paper were analyzed in a simple manner throughout the manuscript with illustrative graphics and in detail concerning the methodology used. A second critical analysis of the science used in this experiment would argue that given the particular task, the authors had to carry a more precise genetic analysis. That is because 16S DNA and COI, are somewhat wide targets to the species level, and are not sufficiently discriminatory (i.e. they are frequently highly preserved) to observe diversity and or genetic polymorphism a micro satellite loci sequencing for example, would offer certainty and speciation to the taxonomical findings (subtler intraspecific genetic variance) and would allow for analysis in population subdivision between areas of habitat. And controversially, the authors do not provide a paragraph that would constitute their choice of this particular genetic analysis as sufficient i.e. stating the methods of analyses characteristics and specificity, nor do they reference previous literature that might have rendered this type of analysis as the gold standard. Moreover, they use phylogeny limits that run back to 1985. To add to these interpretations, the authors admit in their discussion section that the existence of a universal highly conserved COI gene may explain the lack of divergence in corona complex taxa, as has been found for other species.
In total however, the authors are aware and sincere about their limitations and have outlined their contribution to the topic thoroughly and with a sufficient analysis of all pertinent research in the subject.
RELEVANT CHAPTERS
Evolutionary trees the authors try to elucidate the complexity or absence of it, in a marine species, using highly conserved genetic elements, and place one subset of the studied population in a lower evolutionary level than the others. Moreover, they provide evidence of a taxonomical controversy, that allows for a reclassification of whelks in a different family than Melongena.
Migration, drift and nonrandom mating the question of marine migration and the planktonic-form drift constitutes a major argument for this article, where the biogeography of an organism is studied for its selective environmental disparity. Drift is being questioned and no data is found to substantiate the mechanisms of geodisparity in this organism.
Evolutionary analysis of form and function genetic analysis proves low divergence between a selected species that is traditionally confused in its taxonomy due to variability in shell forms.
Phylogenomics and adaptation Phylogenomic methods are being used in this paper to elucidate the place of several subspecies of the Melongena spp. to different subgroups.
Mutation and genetic variation The methods employed by this study do not allow for the identification of mutation, and genetic variation is found to be low within a species.
Evolution at multiple loci This is also a limitation of this study that is not designed to provide with such information for the organism selected.
Life history characters Although geographical disparity is investigated and mapped, there is no explanation to the theory of this organisms particular selection of habitat, although suggestions regarding life history events are hypothesized to play a major role.
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