Genetic Architecture of the Cryptic Species Complex of Acanthocyclops Vernalis (Crustacea: Copepoda). Ii. Crossbreeding Experiments, Cytogenetics, and a Model of Chromosomal Evoloution

Abstract

Abstract. Collectively, populations of Acanthocyclops vernalis, a species complex of freshwater copepods, are remarkably similar as to morphology and DNA content, despite variability in chromosome number. Reproductive isolation had been reported among some populations, but with each new investigation the species boundaries and factors that may influence them appeared less clear. To clarify the pattern of biological species within this group of populations, we adopted a comprehensive approach and examined patterns of reproductive isolation in populations for which morphology, chromosome number, DNA content, and 18S rDNA sequences are known. In this study we established nine isofemale lines from four sites in Wisconsin and performed 266 crosses. Crosses within and among these lines were used to relate the degree of reproductive isolation to chromosome differences and to construct a model to explain the origin and maintenance of chromosome number variability. Different gametic and somatic chromosome numbers were observed among specimens within some isofemale lines. In a few cases, gametes with different haploid numbers were produced by a single female. Matings within isofemale lines always produced at least some reproductively successful replicate crosses (produced viable, fertile offspring). Crosses between lines from the same site showed reduced success relative to within-line crosses. Crosses between populations from distant sites showed limited genetic compatibility, producing viable, fertile F 1 offspring but infertile F 2 adults. One cross between lines with different chromosome numbers (one with 2n ϭ 8 and one with 2n ϭ 10) produced fertile viable offspring, which reproduced for at least 60 generations. These hybrids had either eight or nine chromosomes in the third generation of inbreeding, and eight chromosomes after 20 generations. These hybrids also had reduced nuclear DNA contents at the third generation, a level that persisted through the 20th generation. Successful backcrosses between some hybrids and their parental lines further demonstrated the potential for genetic compatibility among forms with different chromosome numbers. We propose a model in which alterations due to Robertsonian fusions, translocations, and/or loss of chromosomal fragments generate heritable variation, only some of which leads to reproductive isolation. Hence, some of the criteria traditionally used to recognize species boundaries in animals (morphology, DNA content, chromosome number) may not apply to this species complex.

Pacific and Atlantic Lepeophtheirus salmonis (Krøyer, 1838) are allopatric subspecies: Lepeophtheirus salmonis salmonis and L. salmonis oncorhynchi subspecies novo

“…In the present study, we successfully produced F1 and F2 hybrid L. salmonis strains using maternal lines from both the Pacific and the Atlantic. Previous studies of separate conspecific populations of copepods have shown that although defined as single species, the populations may or may not be reproductively compatible [36-38]. In studies of the intertidal copepod Tigriopus californicus it has been shown that “Dobzhansky-Muller incompatibilities” between strains, i.e.…”

Evolutionary History of Chemosensory-Related Gene Families across the Arthropoda

“…They also form the largest biomass of all animals in the world’s oceans, and possibly on the planet ( Hardy 1956 ; Huys and Boxshall 1991 ; Humes 1994 ; Verity and Smetacek 1996 ). Copepods are particularly known to frequently exhibit cases of cryptic speciation, where large genetic distances and reproductive isolation are accompanied by morphological stasis ( Burton 1990 ; Ganz and Burton 1995 ; Edmands 1999 ; Lee 2000 ; Lee and Frost 2002 ; Goetze 2003 ; Grishanin et al 2006 ; Rynearson et al 2006 ; Eyun et al 2007 ; Chen and Hare 2011 ). In the absence of morphological cues and differentiation, it has been hypothesized that speciation in copepods occurs through rapid evolution of chemical sensing ( Snell and Morris 1993 ).…”

Genome architecture evolution in an invasive copepod species complex

“…These chromosomal fusions likely played a crucial role in speciation processes within this species complex 48,57 . Chromosomal fusions are increasingly recognized as important drivers of speciation, especially in systems where closely related species or populations exhibit divergent karyotypes 4,16,47,[70][71][72] . Chromosomal changes can promote speciation events by impeding gene ow between populations, thereby facilitating the accumulation of genetic differences among them 4,16,47,70,72 .…”