Speciation on the Coasts of the New World: Phylogeography and the Evolution of Bindin in the Sea Urchin Genus Lytechinus

Abstract

Abstract. Beginning with E. Mayr's study in 1954, tropical sea urchins have played an important role in studies of speciation in the sea, but what are the processes of cladogenesis and divergence that give rise to new species in this group? We attempt to answer this question in the genus Lytechinus. Unlike the majority of other tropical sea urchin genera, which have circumtropical distributions, Lytechinus is mostly confined to the tropics and subtropics of the New World. We sequenced a region of mitochondrial cytochrome oxidase I and the entire molecule of nuclear bindin (a sperm gamete recognition protein) of nearly all species in the genus, and we assayed isozymes of three partially sympatric closely related species and subspecies. We found that in both mitochondrial DNA (mtDNA) and in bindin the genus Lytechinus is paraphyletic, encompassing Sphaerechinus granularis as the sister species of L. euerces. The multifaceted question of how speciation proceeds requires many kinds of data to be addressed. Mitochondrial DNA (mtDNA) sequences combined with geographical information can provide clues on the order of splitting between clades and on the possible extrinsic barriers that caused the observed patterns. DNA sequences of nuclear loci involved in reproductive isolation can be used to examine the role that divergence in such loci has played in perfecting reproductive isolation. Data on reproductive compatibility can provide information on the degree of completion of the speciation process between putative species. When reproductive isolation is incomplete, independent molecular datasets can identify introgression and prevent incorrect conclusions about species relationships. We carried out a study that combines these types of information in the sea urchin genus Lytechinus.Unlike most genera of shallow water sea urchins that show very wide geographical distributions, Lytechinus is confined almost exclusively to the coasts of America, ranging from California to the Galapagos in the Pacific and from Bermuda to Brazil in the Atlantic (Mortensen 1943). Of the 11 species

Phylogeography of the sand dollar genus Mellita: Cryptic speciation along the coasts of the Americas

“…Molecular phylogenies of regular echinoids (Lessios et al, 1999(Lessios et al, , 2001(Lessios et al, , 2003McCartney et al, 2000;Zigler and Lessios, 2004;Palumbi and Lessios, 2005) have agreed with Mayr's (1954) model of allopatric speciation. However, our data revealed that not all divergence highlighted in this study fits an allopatric model.…”

“…However, one species of Mellita, M. grantii Mortensen, 1948, proved problematic for Mayr (1954) and he chose to ignore it in his analysis, stating that Mortensen (1948) had described M. grantii based on a single specimen from the midst of the range of M. longifissa Michelin, 1858. Mayr's (1954 model of allopatric speciation in echinoids has been supported by molecular phylogenies of regular sea urchins (Lessios et al, 1999(Lessios et al, , 2001(Lessios et al, , 2003(Lessios et al, , 2012McCartney et al, 2000;Zigler and Lessios, 2004;Palumbi and Lessios, 2005), but to-date this hypothesis has not been tested with a molecular phylogeny of a sand dollar genus.…”

Highly contrasted population genetic structures in a host–parasite pair in the Caribbean Sea

“…This pattern was previously detected within the Caribbean in other taxonomic groups (Johnston et al., 2012 ; Purcell et al., 2006; Silberman et al., 1994), as well as in other sea urchins (Lessios, Kane, & Robertson, 2003; Lessios, Kessing, & Pearse, 2001; Lessios et al., 2012; McCartney, Keller, & Lessios, 2000; Zigler & Lessios, 2004). The present study confirms this last observation, not only for mitochondrial DNA, but also for microsatellites.…”

Evolutionary animation: How do molecular phylogenies compare to Mayr's reconstruction of speciation patterns in the sea?

“…In general, phylogenetic relationships at COI and bindin are concordant. Major exceptions are the positions of Lytechinus williamsi and Echinometra insularis in their respective genera (10,12). In Strongylocentrotus, we have included the monotypic genera Allocentrotus and Hemicentrotus because phylogenetic analysis places the species in these genera firmly within the genus Strongylocentrotus (14).…”

“…By this definition, bindin is considered to be evolving quickly in the genera Echinometra, Strongylocentrotus, and Heliocidaris (12,16,18,20) and slowly in Lytechinus, Arbacia, and Tripneustes (10,15,17). Although positive selection could not be statistically demonstrated for any comparison between species of Lytechinus, we consider the bindin of L. williamsi and Lytechinus variegatus variegatus as rapidly evolving, because its divergence is much higher than expected from comparisons of COI between these species.…”