Phylogenetic relationships of african killifishes in the genera aphyosemion and fundulopanchax inferred from mitochondrial dna sequences

Molecular Phylogenetics and EvolutionVol. 11, No. 3, April, pp. 351–360, 1999Article ID mpev.1998.0566, available online at http://www.idealibrary.com on Phylogenetic Relationships of African Killifishes in the
Genera Aphyosemion and Fundulopanchax Inferred
from Mitochondrial DNA Sequences
Department of Biological Sciences, The University of Tulsa, 600 S. College Avenue, Tulsa, Oklahoma 74104 Received May 12, 1997; revised June 17, 1998 Fundulopanchax, and Nothobranchius) and six mono- We have analyzed the phylogenetic relationships of
typic genera (Adamus, Foerschichthys, Fundulosoma, 52 species representing all defined species groups (J. J.
Pronothobranchius, Aphyoplatys, and Episemion). The Scheel, 1990, Atlas of Killifishes of the Old World, 448
composition and relationships of these genera have under- pp.) of the African aplocheiloid fish genera Aphyo-
gone numerous changes as our knowledge of these fishes semion and Fundulopanchax in order to examine their
has grown. The greatest number of changes have occurred interrelationships and to reveal trends of karyotypic
with regard to the genus Aphyosemion Myers 1924.
evolution. The data set comprised 785 total nucleotides
The genus Aphyosemion was originally divided into from the mitochondrial 12S rRNA and cytochrome b
genes. The molecular-based topologies analyzed by
three subgenera: Aphyosemion, Fundulopanchax, and both maximum parsimony and neighbor-joining sup-
Adinops. Those species assigned to Adinops were from port the monophyly of most previously defined species
east Africa and were later removed to the genus groups within these two killifish genera. The genus
Nothobranchius. The remaining species and subse- Aphyosemion is monophyletic except for the nested
quent taxa assigned to these subgenera could be di- position of Fundulopanchax kunzi (batesi group; sub-
vided by distributional criteria and independently by genus Raddaella) within this clade, suggesting that
phenotypic criteria. The vast majority of these fishes this taxon was improperly assigned to Fundulopan-
are found in small streams in the understory of the chax. The remaining Fundulopanchax species sampled
rainforest (Scheel, 1990). The rainforest of equitorial were supported as being monophyletic in most analy-
Africa is cleanly divided into western and eastern ses. Relationships among the species groups in both
blocks by the Dahomey Gap, a strip of savanna habitat genera were not as strongly supported, suggesting that
that extends to the coast in Benin, Togo, and eastern further data will be required to resolve these relation-
Ghana. In 1966, Clausen recognized the distinctiveness ships. Additional sampling from the 16S rRNA gene
of those species west of the Dahomey Gap. Subsequent allowed further resolution of relationships within Fun-
dulopanchax, more specifically identifying the nonan-
workers have identified additional morphological char- nual scheeli group as the basal lineage of this other-
acters that distinguish these western taxa (Zee and wise annual genus. Chromosomal evolution within
Wildekamp, 1995) and recent DNA sequence data Aphyosemion has been episodic, with the evolution of a
(Murphy, 1997, Murphy and Collier, 1997, 1999) clearly reduced n ؍ 9–10 metacentric complement having
identify the western forms as a distinct clade not occurred in multiple, independent lineages. Polarity of
closely related to the eastern taxa. Thus the remaining chromosomal reductions within the elegans species
problem with the genus Aphyosemion involves those group appears to support previous hypotheses concern-
eastern species formerly assigned to the subgenera ing mechanisms of karyotypic change within the genus
Aphyosemion and Fundulopanchax. Aphyosemion.
௠ 1999 Academic Press
The subgenus Fundulopanchax was elevated to ge- neric level by Parenti (1981) based on two characters.
Zee and Wildekamp (1995) dispute the diagnostic value INTRODUCTION
of one of these characters but added four new charac-ters defining Fundulopanchax. Prominent among the African aplocheiloid killifishes are currently as- life history traits that distinguish Aphyosemion and signed to four speciose genera (Aphyosemion, Epiplatys, Fundulopanchax is annualism. Annual fishes (Myers1942, 1955) are those that deposit their eggs in the substrate where they withstand the dessication of an Present address: Laboratory of Genomic Diversity, National Cancer Institute, FCRDC, Frederick, MD 21702.
annual dry season to hatch once the rains resume.
1055-7903/99 $30.00Copyright ௠ 1999 by Academic Press All rights of reproduction in any form reserved.
Members of Fundulopanchax are believed to be annual while members of Aphyosemion are not (Parenti, 1981; Taxa Under Study, Proposed Species Groups and
Their Abbreviations Used in Figures (Scheel, 1990), and
Chromosome complements are relatively well con- Various Subgeneric Names Assigned to Specific Taxa
served in teleostean fishes, particularly within theimmense acanthomorph clade (sensu Johnson and Pat- terson, 1993) in which the predominant diploid karyo- type is 2n ϭ 48 (Sola et al., 1981). Significant deviations from this number have occurred in only a handful of these fish species. One order in particular, the Cypri- bivittatum,* volcanum, sp. LEC 93/27 nodontiformes (killifishes), displays a striking propen- sity for clade-specific karyotypic rearrangement. Per- ahli, australe,* calliurum, celiae haps the best example is the aplocheiloid genus Aphyosemion, which shows more inter- and intraspe- cific chromosomal rearrangements (Scheel, 1990) than aureum, citrinepinnis, coeleste, occe- perhaps any other fish genus. Our knowledge of the types of mechanisms behind karyotypic evolution, and christyi, cognatum (2), decorsei, elegans its potential contribution to speciation within this (3), lamberti, melanopteron, punc- genus, have been hampered by the lack of a phyloge- netic framework for this diverse group.
The specific aim of this work was to use mitochon- drial DNA sequences to assess the monophyly and composition of Aphyosemion and Fundulopanchax, to determine the monophyly of recently proposed subgen- exigoideum, gabunense, louessense, era and species groups (Scheel, 1990; Table 1) within these genera, and to determine the polarity of chromo- somal rearrangements within the molecular phylogeny.
Further, this enlarged data set has allowed further consideration of the origin of annualism (Murphy and Collier, 1997) within these genera. We sampled 36 populations of 32 described and 4 undescribed species of Aphyosemion and 16 species of Fundulopanchax. In total these represent 14 of Scheel’s (1990) 15 species groups. The 15th group, composed of a single species, deltaense, fallax, gulare, schwoiseri, Pronothobranchius kiyawensis, has been subsequently excluded from Aphyosemion on the basis of both morpho- logical and molecular characters (Parenti, 1981; Mur- Note. Generic divisions follow those of Parenti (1981). Numbers in MATERIALS AND METHODS
parentheses denote number of populations sampled. Data from taxamarked with asterisks have previously been reported (Murphy and A list of the taxa examined and their sources is in the Appendix. Mitochondrial DNA was extracted from muscle or liver tissues. Mitochondrial DNA extractions and amplification protocols were performed as previ- ously described (Murphy and Collier, 1996). Some of the sequences have been previously reported (see Table 1).
We sequenced a 360-bp region of the cytochrome b(cytb) gene and a 425-bp region of the 12S rRNA gene.
The primers used were L14724 and H15149 (Kocher et products were purified with 30,000 MW regenerated al., 1989; Meyer et al., 1990) for the cytb segment and cellulose filter devices (Millipore Inc.). Cycle sequenc- L1091 and H1478 (Kocher et al., 1989) for the 12S ing using fluorescent-labeled terminators was per- rRNA segment. Primers 16Sar-L and 16Sbr-H (Palumbi formed using Ampli-Taq FS DNA polymerase (Applied et al., 1991) were used to amplify a region of the 16S Biosystems Inc.). The reactions were purified free of rRNA gene for Fundulopanchax taxa. The new DNA fluorescent terminators using Centri-Sep columns sequences were generated with an automated se- (Princeton Separations) before loading onto a sequenc- quencer (ABI 373 Stretch). Symmetric amplification ing gel (6% Long-Ranger acrylamide, FMC).
PHYLOGENETIC RELATIONSHIPS OF AFRICAN KILLIFISHES Sequences were initially aligned using the program AF002401. The total analyzed data set consisted of 763 CLUSTAL W (Thompson et al., 1994). Manual adjust- bp, following removal of 22 bp of unalignable regions ments were made to the preliminary alignment of the from the 12S rRNA segment. This resulted in 317 rRNA segments. Regions of length variation due to variable sites, 265 being parsimony informative. Nucle- insertions or deletions were omitted from the analyses otide frequencies for the entire data set were A ϭ 31%, when they could not be aligned without making assump- T ϭ 29%, C ϭ 23%, and G ϭ 17% and did not differ tions concerning homology. The complete aligned data significantly between taxa. Transition/transversion ra- sets were analyzed by maximum parsimony (MP) and tios varied from 1.1 to 19.0 in ingroup comparisons, neighbor-joining (NJ; Saitou and Nei, 1987) methods.
with many of the higher ratios (particularly among Parsimony analyses were done with PAUP vers. 3.1.1 closely related taxa) ranging between 5.0 and 10.0.
(Swofford, 1993). In all cases heuristic searches were Parsimony analysis of the data set when all sites used (50 replicates, random addition of taxa, TBR were given equal weight resulted in eight 1806-step branch-swapping). A series of different weighting trees having consistency indices of 0.271 and retention schemes was applied to the parsimony analyses to indices of 0.565. Figure 1 shows the strict consensus of adjust for transitional saturation in increasingly diver- these eight trees. The consistency and retention indices gent comparisons. This phenomenon is well docu- are relatively low, most likely attributable to the large mented in most animal groups, including previous number of taxa analyzed and the resulting increased studies of aplocheiloid phylogeny using mitochondrial probability for homoplasious substitutions at rapidly DNA (Murphy and Collier, 1996, 1997). We employed evolving sites. The members of the genus Aphyosemion the following weighting strategies: (1) all sites given form a strongly supported group (94% of bootstrap equal weight; (2) all sites given equal weight, while replications) which includes Fundulopanchax kunzi excluding first-codon-position leucine transitions and nested within this clade. Bootstrap values are also very all third-codon-position transitions in the cytochrome b high for nodes defining most of Scheel’s species groups segment (conservative substitution, CS parsimony; Ir-win et al., 1991); and (3) all transversions weighted five within Aphyosemion (Fig. 1). The remaining species of times transitions (5:1 parsimony). These last two Fundulopanchax form a monophyletic group, though weighting schemes attempt to resolve deeper diver- this clade is weakly supported by the bootstrap results gences due to the greater conservation of the substitu- (50%). The relationships within Fundulopanchax are tions analyzed. Bootstrap values for all parsimony also poorly resolved by these data, with the exception of analyses were based on 100 heuristic replicates gener- a few interspecific relationships, which corresponded to some of Scheel’s (1990) species groups.
Neighbor-joining analyses were generated in MEGA Weighted parsimony analyses were employed to re- (vers. 1.01; Kumar et al., 1993) with indels or ambigu- solve deeper relationships which might be obscured by ities deleted from the analyses. The Kimura two- transitional saturation. Both weighting schemes (con- parameter correction was used to account for transition servative substitutions and transversions weighted bias (Kimura, 1980). Confidence probabilities (P greater than transitions) produced trees in general Rzhetsky and Nei, 1992) for branches of the NJ tree agreement with the equal-weighted results (Fig. 1b), were assessed with the interior-branch test imple- with most of the species groups within Aphyosemion mented in MEGA (vers. 1.01, Kumar et al., 1993). We being monophyletic, though the relationships between opted to use this method based on recent data suggest- species groups differed somewhat (see Discussion). The ing that the bootstrap conservatively underestimates 5:1 parsimony resulted in two trees of 3520 steps (Fig.
the statistical support for groupings within a topology, 1b). This weight was derived from the higher transition particularly when large numbers of taxa are being rate among closely related taxa (see above). Results analyzed (Sitnikova et al., 1995). All trees were rooted based on weighting transversions between 5 and 10 with members from the genus Nothobranchius (N. kirki times transitions were equivocal. The analysis based on and the monotypic subgenus Pronothobranchius kiya- CS parsimony (eight trees, CI ϭ 0.442, RI ϭ 0.709) wense) and the allied monotypic genus Fundulosoma differed primarily from all other analyses in that most thierryi (Parenti, 1981). A recent molecular analysis of of the Fundulopanchax taxa (exclusive of F. kunzi) were the major aplocheiloid genera demonstrated the sister- not resolved as being monophyletic, collapsing into a group status of Nothobranchius to a monophyletic basal polytomy in the consensus tree (not shown). The Aphyosemion ϩ Fundulopanchax clade (Murphy and bootstrap trees for these latter two weighted analyses Collier, 1997), justifying its use here as an outgroup.
gave similar results (shown in Fig. 1b), strongly support-ing most of the species groups within Aphyosemion, with less resolution between species groups.
The neighbor-joining tree based on Kimura-corrected Sequences obtained for this study have been depos- distances (Fig. 2) was congruent with the strongly ited in GenBank under Accession Nos. AF002284– supported aspects of the parsimony analyses, with (a) Consensus tree of eight equal-length cladograms (TL ϭ 1806; CI ϭ 0.271, RI ϭ 0.565) produced when all sites are given equal weight. Numbers above branches are bootstrap values (Felsenstein, 1985) based on 100 replications. Values below 50% are not shown. Boxesdenote taxa sampled from each of Scheel’s (1990) species groups. Abbreviations for species groups are given in Table 1. (b) Consensus tree oftwo equal-length cladograms (TL ϭ 3519) produced when all transversions were weighted five times greater than transitions. Numbers abovebranches are bootstrap values (Felsenstein, 1985) based on 100 replications. Values in front of the slash are produced by the 5:1transversion/transition weighting scheme. Values following the slash are produced by the bootstrap analysis using conservative substitutions(See text for discussion). Values below 50% are not shown. Boxes denote taxa sampled from each of Scheel’s (1990) species groups.
Abbreviations for species groups are given in Table 1.
most of the differences revolving around branches with To further resolve the relationships with Fundulopan- low statistical support. Similar to parsimony, the spe- chax, an additional 472 bp of sequence data was cies groups within Aphyosemion are strongly supported obtained from the 16S rRNA gene for each taxon. In (PC values), while the relationships between the groups addition, we determined DNA sequences from all three are weaker. Monophyly of Fundulopanchax, exclusive gene segments for two Fundulopanchax species not of F. kunzi, received robust support from the interior- sampled in the initial portion of this study—F. cinna- branch test (Fig. 2). Complete deletion of sites with momeum and F. fallax. Some of the 16S rRNA se- gaps or the use of different distance corrections changed quences have been previously reported (Murphy the topology very little, these differences again being and Collier, 1997). Trees were rooted with two observed among branches with low statistical support.
Aphyosemion species, and the combined data set (1221 PHYLOGENETIC RELATIONSHIPS OF AFRICAN KILLIFISHES Neighbor-joining tree based on Kimura-corrected distances (Kimura, 1980) with pairwise deletion of gaps. Numbers above the branches are confidence probabilities based on the interior-branch test implemented in MEGA vers. 1.01 (Kumar et al., 1993). Bars span taxaassigned to Scheel’s (1990) species groups. Dashed lines connect nonadjacent members of these groups. Abbreviations are given in Table 1. Adistance scale is represented at the bottom.
bp, 330 variable sites, 220 of these parsimony informa- define monophyletic lineages which support Scheel’s tive) was analyzed by three parsimony methods (all groupings: bivittatum, calliurum, cameronense, coe- sites equal weight, conservative substitutions only, and leste, exiguum, and striatum groups.
transversions weighted five times transitions), neighbor- The georgiae species group [the subgenus Diapteron joining, and, as allowed by the smaller size of the data of Huber and Seegers (1977)] is represented in the set, maximum likelihood (fastDNAML; Olsen et al., combined dataset by a single taxon—A. cyanostictum. 1994). A 6-bp region of the 12S rRNA segment was Two additional species from this group, abacinum and deleted due to ambiguity in alignment. Weighted parsi- georgiae, were sampled; however, the cytochrome b mony, neighbor-joining, and maximum likelihood analy- primers were unsuccessful with DNA from these taxa.
ses all produced the topology shown in Fig. 3. Equal- The results from 12S rRNA gene alone (not shown) weighted parsimony generated a single tree congruent demonstrated that all three species comprise a mono- with Fig. 3, with the exception of F. amieti being placed phyletic group, with abacinum being basal to the other basal in the gulare-group clade. The conservative sub- two. These data also resolved the georgiae group as the stitution parsimony tree placed the scheeli group sister sister taxon to the exiguum group, as did the combined to the gardneri-group/F. ndianum clade.
dataset. The nested placement of this clade within, andnot outside of, Aphyosemion does not support Seeger’s(1980) suggestion of full generic rank for this group.
DISCUSSION
Two other Aphyosemion subgenera [Chromaphyo- semion (Radda, 1977) and Kathetys (Huber, 1977)] are Phylogenetic Relationships within Aphyosemion also supported here by the apparent monophyly of the These molecular results are highly concordant with corresponding bivittatum and exiguum species groups.
previous species-group definitions of the genus Aphyo- The subgenus Mesoaphyosemion (Radda, 1977), which semion, created on the basis of karyology, meristics, includes the cameronense, calliurum, coeleste, elegans, and geographical distributions (Scheel, 1990). Initial scheeli, and striatum groups, is clearly not a monophy- sampling from the following species groups appear to Phylogenetic hypothesis for the genus Fundulopanchax based on the expanded (cytochrome b ϩ 12S rRNA ϩ 16S rRNA) data set, using Aphyosemion as an outgroup. Weighted parsimony (TL ϭ 1682), neighbor-joining with Kimura (1980) distances, and maximumlikelihood (Ϫln Likelihood ϭ Ϫ6010.09303) analyses all produced this same topology. The equal-weighted and conservative substitutionparsimony trees are discussed in the text. Numbers above the branches are bootstrap values (500 replicates) compatible with equal-weightedparsimony, conservative substitution parsimony, and 5:1 (Tv:Ts) weighted parsimony. Numbers below the branches are the results of theinterior-branch test (Rzhetsky and Nei, 1992) of the neighbor-joining tree, implemented in MEGA (Kumar et al., 1993).
PHYLOGENETIC RELATIONSHIPS OF AFRICAN KILLIFISHES The placement of the annual Fundulopanchax kunzi circumpeduncular scale rows characteristic of most within a larger clade of nonannual Aphyosemion spe- other Fundulopanchax is also characteristic of the cies is quite unexpected. This species is part of a small scheeli group (Zee and Wildekamp, 1995). Given our group (batesi group) (Scheel, 1990) distributed in up- topology for Fundulopanchax (Fig. 3) it would appear land habitats of eastern Cameroon and western Gabon that 16 or more cp scales is diagnostic for Fundulopan- disjunct from the usual coastal ranges of the remaining chax, being secondarily reduced in the derived arnoldi species of Fundulopanchax. The batesi group is, how- ever, sympatric in some areas with the groups for whichthe molecular data suggest phylogenetic similarity.
Recently, Zee and Wildekamp (1994) presented new Basal members of both Aphyosemion (calliurum morphological characters defining Fundulopanchax group) and Fundulopanchax (scheeli group) occupy the which are absent in members of the batesi group eastern region of Cameroon, suggesting that this region (subgenus Raddaella). Chorionic puncti, present on the may have been the center of diversification of these two surface of eggs of all annual Fundulopanchax species, genera. The genus Aphyosemion subsequently diversi- are lacking in the batesi group. All species of Fundulo- fied to the east with the terminal elegans group rela- panchax (with the exception of the arnoldi group) have tively recently filling the Zaire basin. The genus Fundu- a mean of 16 or more circumpeduncular scales (cp lopanchax diversified westward in essentially coastal scales), while all species of Aphyosemion sampled (in- habitats with the gardneri group expanding inland to cluding the batesi group) have a mean of 14.2 or fewer fill much of the area of Nigeria. Fundulopanchax cp scales. Together, these two morphological characters walkeri is the only member of the group to have crossed support the placement of the batesi group within the Dahomy Gap to occupy western habitat in Ghana and Togo. Given its rather terminal position within the The interrelationships between the Aphyosemion spe- combined Aphyosemion/Fundulopanchax clade, this cies groups are less apparent with these data. Our has been a relatively recent event. As such, it does not results distinguish two major components within the invalidate the presumption of the epicontinental seas genus: (1) a strongly supported monophyletic clade being the viacariant event separating the eastern and containing the batesi, cameronense, coeleste, and el- western aplocheiloid taxa (Murphy and Collier, 1997; egans groups, plus the ungrouped A. labarrei; and (2) a basal grade composed of the bivittatum, calliurum,exiguum, georgiae, and striatum groups. The former group is centered primarily around the Congo Riversystem and its neighboring drainages in Gabon while The suite of traits referred to as annualism includes the latter group is centered primarily in Cameroon and behavioral components (bottom spawning), morphologi- cal components (enlarged dorsal and anal fins of males The neighbor-joining tree also supports a basal bifur- and features of chorion structure), and developmental cation between the calliurum/bivittatum/exiguum/ components (embryonic diapauses). Within the subor- georgiae groups and the remaining species groups, der Aplocheiloidei, annualism has been hypothesized to though the monophyly of the former group is supported have arisen once and then to have been subsequently by a very short internode, which receives no support lost and regained (Murphy and Collier, 1997). The more from the interior-branch test. However, when members detailed phylogenetic analysis presented here, with a of Fundulopanchax are used as an outgroup for the NJ unique annual species (F. kunzi) nested within an analysis (data not shown) the calliurum group is found otherwise nonannual clade and the nonannual scheeli as the most basal Aphyosemion clade, similar to parsi- group being the basal group of the otherwise faculta- tively annual Fundulopanchax, suggests that annual-ism is evolutionarily more plastic than once thought.
Phylogenetic Relationships within Fundulopanchax In contrast to Aphyosemion, only two of Scheel’s Fundulopanchax species groups are supported by the Acanthomorph fishes have extremely conserved current molecular data. One of these, the scheeli group, karyotypes, with the vast majority of taxa having a is the lone nonannual taxon within Fundulopanchax, diploid number of 44–48 chromosomes (Sola et al., inhabiting a restricted range in Cameroon and south- 1981). This number is particularly well conserved eastern Nigeria. The monophyly of this distinctive set within the speciose Percomorpha. While there are a few of species is strongly supported (bootstrap ϭ 100% all scattered examples of significant reductions in number analyses, PC ϭ 0.99) and the addition of the 16S rRNA throughout the acanthomorph fishes, the Order Cypri- segment stabilizes the basal position of the scheeli nodontiformes exhibits more documented inter- and group in weighted parsimony, neighbor-joining, and intraspecific variability than perhaps any other fish maximum likelihood analyses. The elevated number of group of equal phylogenetic diversity. The suborder Cyprinodontoidei has well-conserved chromosome num-bers, with the only significant examples of reductioncoming from the Goodeidae (Turner et al., 1985). Withinthe sister-suborder Aplocheiloidei we see an increasedpropensity for karyotypic reduction, the Neotropicalclade Rivulidae showing a few sporadic cases (n ϭ 10for Pterolebias longipinnis, n ϭ 16–17 in a handful oftaxa) amid a larger trend of n ϭ 22–24 chromosomes(Scheel, 1972, 1990; Garcia et al., 1993; Collier, unpub-lished data).
The African genera show by far the greatest karyologi- Distribution of karyotypic data onto the phylogeny for the cal variability—the most striking case found within the monophyletic Zaire-basin components of the elegans group. Karyo-typic information for each species is from Scheel (1990), except genus Aphyosemion (Scheel, 1990). Of the eight defined ‘‘elegans’’ Epoma and ‘‘cognatum’’ Lake Fwa (Collier, unpublished species groups, three (bivittatum, calliurum, and el- egans) contain taxa with karyotypes ranging from n ϭ18 to n ϭ 9 or 10 (Fig. 4). The remaining species groupsshow much less variability in chromosome number and such reduced chromosome numbers: the bristlemouth morphology. One of the more notable findings from this Gonostoma bathyphilum (Gonostomatidae), n ϭ 6; the study is that the species groups showing reduced gouramie Sphaerichthys osphromonoides (Belonti- karyotypes are not phylogenetically restricted. Rather, idae), n ϭ 8; (Sola et al., 1981).
this propensity toward reduced karyotypes has oc- Within Aphyosemion a consensus topology can be curred multiple times in Aphyosemion. A similar ex- constructed based on the relationships depicted in both treme reduction in haploid number has occurred in parsimony and neighbor-joining analyses (Fig. 4). One Nothobranchius rachovii (n ϭ 9/18 arms; Scheel, 1972, observation is that a karyotype of 19–20 haploid chro- 1990). This trend is particularly striking because the mosomes is found in all of the basal species groups populations having n ϭ 9 to 10 in Aphyosemion (14) (bivittatum, calliurum, exiguum, georgiae, and stria- outnumber all other nonaplocheiloid teleosts having tum), while the monophyletic ‘‘eastern’’ groups (batesi,cameronense, coeleste, and elegans) have an apparentlyancestral upper limit of n ϭ 17 chromosomes. Thepresence of n ϭ 20 chromosomes in the basal groupssuggests that it might be the ancestral karyotype forthe genus and that reduction has occurred in eachgroup. Determining the polarity of chromosome changeswithin most of the species groups is currently notpossible without more complete sampling. However,Scheel’s study of the calliurum group suggests that theNyong-North population of A. ahli (n ϭ 20, acrocentricelements) is ancestral and gave rise to the remainingkaryotypes of reduced number in the group (Scheel,1990). The more derived taxon-pair calliurum andceliae have reduced karyotypes (10 populations, alln Յ 12), and the grouping of these two taxa in allanalyses, together with some analyses showing ahliand australe in basal positions, supports this generaltrend towards reduced karyotypes.
Scheel hypothesized that aplocheiloid karyotypes have evolved via two major mechanisms: pericentricinversions and centric fusions (Scheel, 1972, 1990).
Pericentric inversions would move metacentric centro-meres into terminal positions. Acrocentric chromo-somes could then undergo centric fusions to return thecomplement to one of oversized metacentric elements Hypothesis for chromosomal evolution within the genus and a reduced number of chromosomes. If this hypoth- Aphyosemion. Numbers preceding the slash are haploid chromosome esis is correct, we would expect to see basal taxa having numbers; numbers following the slash indicate the number of chromo- higher chromosome numbers and more acrocentric some arms in the haploid complement. The range of values for each elements, while terminal, derived taxa would have terminal taxon are given on the respective branches. Karyologicaldata are derived from Scheel (1990). See text for discussion.
lower haploid numbers with symmetrical (metacentric) PHYLOGENETIC RELATIONSHIPS OF AFRICAN KILLIFISHES complements. Our sampling of several described el- ACKNOWLEDGMENTS
egans group members presents us with a template fortesting this hypothesis of chromosomal evolution within We thank the following people for supplying various specimens, Aphyosemion. Figure 5 shows the NJ topology of the either wild-caught or aquarium strains: Brian Abbott, Jim Gasior, Liz elegans group. Analysis of this group alone produces a Hutchings, Mike Kromrez, Monty Lehman, Robert Langton, Dick similar topology with both parsimony (all sites equal Martino, Leonard McCowiac, Craig Rees, Ralph Tepedino, PeteTirbak, Bob Tornatore, Darrell Ullisch, Hans Van Es, and Ed Warner.
weight) and NJ methods. The distribution of karyotype Many of the aquarium strains were obtained through the American information onto this tree shows a gradual transition Killifish Association. We are indebted to this group of naturalists/ from n ϭ 15, mostly acrocentric elements in A. melanop- hobbyists and various other international killifish organizations for teron, to both elegans populations having n ϭ 10/18 their past and continuing preservation of these fishes.
arms in the upper lineage. All taxa in the bottomlineage exhibit completely symmetrical complements ofn ϭ 9/18 arms, though there appears to be a rare REFERENCES
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