sixteen and thirty-seven times greater than intraspecific ones respectively, while
the trans-isthmian distance in Diadema was twenty times smaller than for the
Echinometra. Lessios (1979) proposed that the most plausible explanation was that
the molecules used in this investigation were evolving under the influence of
natural selection.
In 1981(b) Lessios combined this molecular work with morphometric data
using twenty morphological measurements. The morphometric data showed that
differentiation between members of each geminate pair are not significantly different from local variation within each species. However, the ratio of inter to
intraspecific Mahalanobis distance was congruent with the molecular data, being
lowest in Diadema, intermediate in Eucidaris, and highest in Echinometra.
Morphological rates of divergence were found to be more rapid in sympatric
species, possibly due to habitat separation between congeners (Lessios 1981b).
Bermingham and Lessios (1993) used mitochondrial DNA (mtDNA) and
isozymes in these sea urchin species to measure the degree of divergence. Transisthmian isozyme divergence in the geminate pairs differed by an order of magnitute, while mtDNA divergence was equivalent in all pairs demonstrating that
mtDNA can provide accurate estimates of time since separation (Bermingham and
Lessios 1993).
After the mass mortality of D. antillarum in the Caribbean (see Sect. 4.6.
Threats to echinoderms), Lessios (1985b) looked at the genetic consequences of
the restriction in the genetic structure of populations. His results showed no loss of
genetic variability to that recorded in the same populations of D. antillarum in
1977. Lessios suggested that organisms with planktonic larvae are likely to recover
from local extinction events without significant losses of their genetic variability,
as larval exchange allows re-seeding of affected areas by recruits not exposed to
the bottleneck.
A phylogenetic survey of sea urchin retroviral-like (SURL) retrotransposable
elements in 33 species of echinoid (regular sea urchins, sand dollars, and heart
urchins) was conducted by Gonzalez and Lessios (1999). High ratios of synonymous to nonsynonymous substitutions suggest that the reverse transcriptase of the
elements is under strong purifying selection (Gonzalez and Lessios 1999). They
report that despite the predominance of vertical transmission, sequence similarity
of 83–94 % for SURL elements from hosts that have been separated for 200 Myr
suggests four cases of apparent horizontal transfer between the ancestors of the
extant echinoid species. In three additional cases, elements with identical RT
sequences were found in sea urchin species separated for a minimum of 3 Myr.
Thus, horizontal transfer plays a role in the evolution of this retrotransposon
family (Gonzalez and Lessios 1999).
Cytochrome oxidase I (COI) divergence was assessed in Atlantic and eastern
Pacific Echinometra by McCartney et al. (2000), using the closure of the Isthmus
of Panama to date cladogenic events. They reported that the Atlantic species
E. lucunter and E. viridis diverged 1.27–1.62 mya at a time in the Pleistocene
when sea levels fell and Caribbean coral speciation and extinction rates were high.
The fact that these species split so recently, yet do not hybridize, demonstrates that
4 Echinoderm Diversity in Panama
125
the trans-isthmian distance in Diadema was twenty times smaller than for the
Echinometra. Lessios (1979) proposed that the most plausible explanation was that
the molecules used in this investigation were evolving under the influence of
natural selection.
In 1981(b) Lessios combined this molecular work with morphometric data
using twenty morphological measurements. The morphometric data showed that
differentiation between members of each geminate pair are not significantly different from local variation within each species. However, the ratio of inter to
intraspecific Mahalanobis distance was congruent with the molecular data, being
lowest in Diadema, intermediate in Eucidaris, and highest in Echinometra.
Morphological rates of divergence were found to be more rapid in sympatric
species, possibly due to habitat separation between congeners (Lessios 1981b).
Bermingham and Lessios (1993) used mitochondrial DNA (mtDNA) and
isozymes in these sea urchin species to measure the degree of divergence. Transisthmian isozyme divergence in the geminate pairs differed by an order of magnitute, while mtDNA divergence was equivalent in all pairs demonstrating that
mtDNA can provide accurate estimates of time since separation (Bermingham and
Lessios 1993).
After the mass mortality of D. antillarum in the Caribbean (see Sect. 4.6.
Threats to echinoderms), Lessios (1985b) looked at the genetic consequences of
the restriction in the genetic structure of populations. His results showed no loss of
genetic variability to that recorded in the same populations of D. antillarum in
1977. Lessios suggested that organisms with planktonic larvae are likely to recover
from local extinction events without significant losses of their genetic variability,
as larval exchange allows re-seeding of affected areas by recruits not exposed to
the bottleneck.
A phylogenetic survey of sea urchin retroviral-like (SURL) retrotransposable
elements in 33 species of echinoid (regular sea urchins, sand dollars, and heart
urchins) was conducted by Gonzalez and Lessios (1999). High ratios of synonymous to nonsynonymous substitutions suggest that the reverse transcriptase of the
elements is under strong purifying selection (Gonzalez and Lessios 1999). They
report that despite the predominance of vertical transmission, sequence similarity
of 83–94 % for SURL elements from hosts that have been separated for 200 Myr
suggests four cases of apparent horizontal transfer between the ancestors of the
extant echinoid species. In three additional cases, elements with identical RT
sequences were found in sea urchin species separated for a minimum of 3 Myr.
Thus, horizontal transfer plays a role in the evolution of this retrotransposon
family (Gonzalez and Lessios 1999).
Cytochrome oxidase I (COI) divergence was assessed in Atlantic and eastern
Pacific Echinometra by McCartney et al. (2000), using the closure of the Isthmus
of Panama to date cladogenic events. They reported that the Atlantic species
E. lucunter and E. viridis diverged 1.27–1.62 mya at a time in the Pleistocene
when sea levels fell and Caribbean coral speciation and extinction rates were high.
The fact that these species split so recently, yet do not hybridize, demonstrates that
4 Echinoderm Diversity in Panama
125