Heterochrony and modularity in the degeneration of maxillopodan nauplius eyes

Abstract

Eye degeneration is a general evolutionary tendency shown in many animal groups that are adapted to dark environments. Thus far, the degenerative process has only been discussed within a few taxonomic units, in terms of both evolution and development, and more studies are needed to deal adequately with this area of ostracod biology. The ostracods and copepods examined in the present study are small crustaceans that are widely diversified in the aquatic environment, and also in interstitial environments, and their 'nauplius eyes' (primitive eye in Crustacea; typically composed of three or four ocelli) show various degrees of degeneration. The ultrastructure and ontogeny of their degenerated nauplius eyes are described for the first time in the present study, using transmission electron microscopy. According to our observations, two morphotypes for degenerative nauplius eyes (i.e. 'tapetal-less form' and 'pigment reduced form') are found in both taxa. The first description of the embryogenesis of normal (none-degenerated) nauplius eyes of surface species is also provided. From a comparison between the embryogenesis of normal nauplius eyes and the postembryogenic development of the 'tapetal-less form', it is strongly suggested that the 'tapetal-less form' is derived by paedomorphic evolution. On the basis of our observations, as well as on previous studies, we propose the hypothesis that modularity, in the form of hierarchical interactions, exists in the nauplius eye (i.e. the tapetal cells constitute an independent developmental module to be distinguished from other developmental or functional modules, including both the pigmented cells and the sensory cells). According to our hypothesis on the nauplius eyes, we also discuss the possibility that the degenerative process is constrained within the general developmental and functional context.

Molecular phylogeny of interstitial Polycopidae ostracods (Crustacea) and descriptions of a new genus and four new species

“…Like other interstitial animals, these ostracods often show specialized morphological characters (Hartmann, ; Maddocks, ; Gottwald, ; Higashi & Tsukagoshi, ), e.g. they are much smaller than their surface‐dwelling relatives (Danielopol & Bonaduce, ; Yamada & Tanaka, ) and have reduced or absent eyes (Kaji & Tsukagoshi, ). Reduction of body size often results in convergent evolution of many characters both of the carapace and soft parts, and therefore it is sometimes hard to reveal the true phylogenetic relationships amongst closely related taxa based on morphological characters alone (Gottwald, ).…”

Heterochrony and developmental modularity of cranial osteogenesis in lipotyphlan mammals

“…From studies on the genetic, developmental and functional modules across vertebrates, it has been implied that developmental modularity may provide insights into processes of morphological evolution [ 24 - 34 ]. Most studies have focused on the physical relationships among functionally- or developmentally-related structures, and there are only a few studies relating developmental timing to the concept of modularity [ 9 - 11 , 35 ].…”

The lunule of caligid copepods: an evolutionarily novel structure

“…Sections were obtained using an ultramicrotome and stained with 1% potassium permanganate solution in distilled water for 2 min and lead citrate for 3 min. These methods were adopted from Kaji and Tsukagoshi (2010).…”